diff --git a/tag.c b/tag.c
index b098eb7..0f44d7e 100644
--- a/tag.c
+++ b/tag.c
@@ -674,7 +674,7 @@ tags_expand_template(const char *template, const struct tag_set *tags)
             const long max = tag->max(tag);
             long value = kind == VALUE_MIN ? min : max;
 
-            const char *fmt;
+            const char *fmt = NULL;
             switch (format) {
             case FMT_DEFAULT: fmt = zero_pad ? "%0*ld" : "%*ld"; break;
             case FMT_HEX:     fmt = zero_pad ? "%0*lx" : "%*lx"; break;
@@ -704,6 +704,8 @@ tags_expand_template(const char *template, const struct tag_set *tags)
             }
             }
 
+            assert(fmt != NULL);
+
             char str[24];
             snprintf(str, sizeof(str), fmt, digits, value);
             sbuf_append(&formatted, str);