diff --git a/lib/isc/mem.c b/lib/isc/mem.c
index 767f910c13..a68d7d6766 100644
--- a/lib/isc/mem.c
+++ b/lib/isc/mem.c
@@ -981,6 +981,7 @@ isc__mem_free(isc_mem_t *ctx, void *ptr FLARG) {
 	size_t size = 0;
 
 	REQUIRE(VALID_CONTEXT(ctx));
+	REQUIRE(ptr != NULL);
 
 	size = sallocx(ptr, 0);