diff --git a/check/fixes.frm b/check/fixes.frm
index fa21fc7b..61e9c522 100644
--- a/check/fixes.frm
+++ b/check/fixes.frm
@@ -5141,3 +5141,17 @@ Global F1 = E{`i' % 10}*E2*E3*E4;
 .end
 assert succeeded?
 *--#] Issue808 :
+*--#[ Issue242 :
+* An #endif without a matching #if must be diagnosed deterministically.
+* Previously the preprocessor peeked at the uninitialized sentinel slot of
+* the PreTypes stack (PreTypes[0] with NumPreTypes == 0), so the behavior
+* depended on heap garbage (Valgrind: "Conditional jump or move depends on
+* uninitialised value" in DoEndif, reported at
+* https://github.com/form-dev/form/issues/242).
+#ifdef `A'
+#message A is defined
+#endif
+#endif
+.end
+assert preprocess_error?("#endif without corresponding #if")
+*--#] Issue242 :
diff --git a/sources/startup.c b/sources/startup.c
index 126b7d3b..b444f498 100644
--- a/sources/startup.c
+++ b/sources/startup.c
@@ -1306,6 +1306,13 @@ void StartVariables(void)
 	AP.MaxPreTypes = 10;
 	AP.NumPreTypes = 0;
 	AP.PreTypes = (int *)Malloc1(sizeof(int)*(AP.MaxPreTypes+1),"preprocessor types");
+/*
+	The sentinel slot must be initialized: with an empty stack the
+	preprocessor termination handlers (DoEndif, DoElse, DoElseif, ...)
+	peek at PreTypes[NumPreTypes] == PreTypes[0], and Malloc1 does not
+	zero the memory.
+*/
+	AP.PreTypes[0] = PRETYPENONE;
 	AP.inside.buffer = 0;
 	AP.inside.size = 0;