diff --git a/programs/escape.c b/programs/escape.c
--- a/programs/escape.c
+++ b/programs/escape.c
@@ -23,6 +23,29 @@
 #include "common.h"
 #include "escape.h"
 
+#include <unistd.h>
+
+/* ESCAPE-INVARIANT check (scripts/escape_invariant.diff, instrumentation
+ * only): strcat (d, s) appends s at the first NUL at or after d, and the
+ * escape loops then advance d by strlen (s), which is right only if the
+ * output is NUL-terminated at d. Report and exit before a strcat whose
+ * destination is not terminated at d. */
+static char *
+escape_checked_strcat (char *d, const char *s, const char *func)
+{
+  if (*d != 0)
+    {
+      fprintf (stderr,
+               "ESCAPE-INVARIANT output not NUL-terminated at the strcat "
+               "position in %s (byte 0x%02x)\n",
+               func, (unsigned char)*d);
+      fflush (stderr);
+      _exit (86);
+    }
+  return strcat (d, s);
+}
+#define strcat(d, s) escape_checked_strcat ((d), (s), __func__)
+
 char * ATTRIBUTE_MALLOC
 htmlescape (const char *restrict src, const int cp)
 {