diff --git a/cf-check/db_structs.h b/cf-check/db_structs.h
index 17ba14e73b..8a92eed4c7 100644
--- a/cf-check/db_structs.h
+++ b/cf-check/db_structs.h
@@ -125,7 +125,7 @@ static const char *const observable_strings[] =
 };
 
 // Not the actual count, just the room we set aside in struct (and LMDB):
-#define CF_OBSERVABLES 100
+#define CF_OBSERVABLES 300
 
 typedef struct Averages
 {
diff --git a/cf-check/dump.c b/cf-check/dump.c
index 6ded3c8bbd..e8af8eb8a3 100644
--- a/cf-check/dump.c
+++ b/cf-check/dump.c
@@ -162,10 +162,14 @@ static void print_struct_lock_data(
 static void print_struct_averages(
     const MDB_val value, const bool strip_strings, const char *tskey_filename)
 {
-    assert(sizeof(Averages) == value.mv_size);
-    if (sizeof(Averages) != value.mv_size)
+    // A record written before an increase of CF_OBSERVABLES is shorter than the
+    // current struct (e.g. after a package upgrade, until cf-monitord rewrites
+    // it). Accept any size up to the full struct and copy into a zeroed struct
+    // so the un-stored trailing slots read back as zero.
+    assert(value.mv_size <= sizeof(Averages));
+    if (value.mv_size > sizeof(Averages))
     {
-        // Fall back to simple printing in release builds:
+        // Unexpected: larger than the struct. Fall back to simple printing.
         print_json_string(value.mv_data, value.mv_size, strip_strings);
     }
     else
@@ -173,7 +177,8 @@ static void print_struct_averages(
         // TODO: clean up Averages
         char **obnames = NULL;
         Averages averages;
-        memcpy(&averages, value.mv_data, sizeof(averages));
+        memset(&averages, 0, sizeof(averages));
+        memcpy(&averages, value.mv_data, value.mv_size);
         const time_t last_seen = averages.last_seen;
 
         obnames = GetObservableNames(tskey_filename);
diff --git a/cf-check/observables.c b/cf-check/observables.c
index 857c6a192d..2619ad1dce 100644
--- a/cf-check/observables.c
+++ b/cf-check/observables.c
@@ -67,10 +67,23 @@ char **GetObservableNames(const char *ts_key_path)
 
             if (fgets(line, CF_MAXVARSIZE, f) == NULL)
             {
-                Log(LOG_LEVEL_ERR,
-                    "Error trying to read ts_key from file '%s'. (fgets: %s)",
-                    filename,
-                    GetErrorStr());
+                if (ferror(f))
+                {
+                    Log(LOG_LEVEL_ERR,
+                        "Error trying to read ts_key from file '%s'. (fgets: %s)",
+                        filename,
+                        GetErrorStr());
+                }
+                /* The ts_key has fewer entries than CF_OBSERVABLES -- e.g. it
+                 * was written by an older agent with a smaller CF_OBSERVABLES.
+                 * Fill the remaining names so the returned array keeps its
+                 * documented non-NULL guarantee (callers index all
+                 * CF_OBSERVABLES entries). */
+                for (int j = i; j < CF_OBSERVABLES; ++j)
+                {
+                    snprintf(buf, CF_MAXVARSIZE, "spare[%d]", j);
+                    temp[j] = xstrdup(buf);
+                }
                 break;
             }
 
diff --git a/cf-check/observables.h b/cf-check/observables.h
index 1c8d932b96..dd33be3d56 100644
--- a/cf-check/observables.h
+++ b/cf-check/observables.h
@@ -4,7 +4,7 @@
 #include <sequence.h>
 
 // copy of libpromises/cf3.defs.h, TODO refactor
-#define CF_OBSERVABLES 100
+#define CF_OBSERVABLES 300
 
 char **GetObservableNames(const char *ts_key_path);
 
diff --git a/libpromises/cf3.defs.h b/libpromises/cf3.defs.h
index fd53bf8a68..c2e060c055 100644
--- a/libpromises/cf3.defs.h
+++ b/libpromises/cf3.defs.h
@@ -148,7 +148,7 @@ typedef enum
 #define CF_MEASURE_INTERVAL (5.0*60.0)
 #define CF_SHIFT_INTERVAL (6*3600)
 
-#define CF_OBSERVABLES 100
+#define CF_OBSERVABLES 300
 
 /* Special exit codes */
 #define EC_EVAL_ABORTED 6       /* like SIGABRT, but signal exit codes are 120+SIG */
diff --git a/libpromises/constants.c b/libpromises/constants.c
index 7562b5aa64..8ade1ced25 100644
--- a/libpromises/constants.c
+++ b/libpromises/constants.c
@@ -179,4 +179,208 @@ const char *const OBSERVABLES[CF_OBSERVABLES][2] =
     {"spare", "unused"},
     {"spare", "unused"},
     {"spare", "unused"},
+    /* ENT-6511: slots 100-299, added when CF_OBSERVABLES was raised
+       100->300. Every slot must have a name entry: cf-monitord's
+       GetObservable() falls back to OBSERVABLES[i] for unregistered
+       slots, so a table shorter than CF_OBSERVABLES => NULL deref. */
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
+    {"spare", "unused"},
 };
diff --git a/tests/unit/mon_cpu_test.c b/tests/unit/mon_cpu_test.c
index d8eafadd74..3399fb08c8 100644
--- a/tests/unit/mon_cpu_test.c
+++ b/tests/unit/mon_cpu_test.c
@@ -52,7 +52,7 @@ static double GetCpuStat()
 
 void test_cpu_monitor(void)
 {
-    double cf_this[100];
+    double cf_this[300];
     double dq1 = GetCpuStat();
     if (dq1 == -1.0)
     {
diff --git a/tests/unit/mon_load_test.c b/tests/unit/mon_load_test.c
index c25789f9ac..de143e0690 100644
--- a/tests/unit/mon_load_test.c
+++ b/tests/unit/mon_load_test.c
@@ -6,7 +6,7 @@
 
 void test_load_monitor(void)
 {
-    double cf_this[100];
+    double cf_this[300];
     MonLoadGatherData(cf_this);
     double load1[2] = {0,0};
     double load2[2] = {0,0};
diff --git a/tests/unit/mon_processes_test.c b/tests/unit/mon_processes_test.c
index 50b59fc620..fff48dd224 100644
--- a/tests/unit/mon_processes_test.c
+++ b/tests/unit/mon_processes_test.c
@@ -100,7 +100,7 @@ static bool GetSysUsers( int *userListSz, int *numRootProcs, int *numOtherProcs)
 
 void test_processes_monitor(void)
 {
-    double cf_this[100] = { 0.0 };
+    double cf_this[300] = { 0.0 };
     MonProcessesGatherData(cf_this);
     MonProcessesGatherData(cf_this);
     MonProcessesGatherData(cf_this);