diff --git a/changes-entries/substitute-pattern-oob-read.txt b/changes-entries/substitute-pattern-oob-read.txt
new file mode 100644
index 0000000000..55eea6f041
--- /dev/null
+++ b/changes-entries/substitute-pattern-oob-read.txt
@@ -0,0 +1,2 @@
+  *) mod_substitute: Fix crash or misbehaviour when loading a Substitute
+     directive with a missing closing delimiter.  [Joe Orton]
diff --git a/modules/filters/mod_substitute.c b/modules/filters/mod_substitute.c
index ada62a6da1..2533d7dcd0 100644
--- a/modules/filters/mod_substitute.c
+++ b/modules/filters/mod_substitute.c
@@ -679,7 +679,7 @@ static const char *set_pattern(cmd_parms *cmd, void *cfg, const char *line)
     if (delim)
         from = ++ourline;
     if (from) {
-        if (*ourline != delim) {
+        if (*ourline && *ourline != delim) {
             while (*++ourline && *ourline != delim);
         }
         if (*ourline) {
@@ -688,7 +688,7 @@ static const char *set_pattern(cmd_parms *cmd, void *cfg, const char *line)
         }
     }
     if (to) {
-        if (*ourline != delim) {
+        if (*ourline && *ourline != delim) {
             while (*++ourline && *ourline != delim);
         }
         if (*ourline) {