diff --git a/png.c b/png.c
index 757c755f9..a076ba7b6 100644
--- a/png.c
+++ b/png.c
@@ -752,7 +752,7 @@ png_convert_to_rfc1123_buffer(char out[29], png_const_timep ptime)
 
    {
       size_t pos = 0;
-      char number_buf[5]; /* enough for a four-digit year */
+      char number_buf[5] = {0}; /* enough for a four-digit year */
 
 #     define APPEND_STRING(string) pos = png_safecat(out, 29, pos, (string))
 #     define APPEND_NUMBER(format, value)\