diff --git a/main/file.c b/main/file.c
index 6631112c4f..164560312e 100644
--- a/main/file.c
+++ b/main/file.c
@@ -1365,7 +1365,7 @@ char *ast_format_str_reduce(char *fmts)
 	char *fmts_str[AST_MAX_FORMATS];
 	char *stringp, *type;
 	char *orig = fmts;
-	int i, j, x, found;
+	int i, j, x, first, found;
 	int len = strlen(fmts) + 1;
 
 	if (AST_LIST_LOCK(&formats)) {
@@ -1392,11 +1392,19 @@ char *ast_format_str_reduce(char *fmts)
 	}
 	AST_LIST_UNLOCK(&formats);
 
+	first = 1;
 	for (i = 0; i < x; i++) {
+		/* ignore invalid entries */
+		if (!fmts_ptr[i]) {
+			ast_log(LOG_WARNING, "ignoring unknown format '%s'\n", fmts_str[i]);
+			continue;
+		}
+
 		/* special handling for the first entry */
-		if (i == 0) {
+		if (first) {
 			fmts += snprintf(fmts, len, "%s", fmts_str[i]);
 			len -= (fmts - orig);
+			first = 0;
 			continue;
 		}
 
@@ -1415,6 +1423,11 @@ char *ast_format_str_reduce(char *fmts)
 		}
 	}
 
+	if (first) {
+		ast_log(LOG_WARNING, "no known formats found in format list (%s)\n", orig);
+		return NULL;
+	}
+
 	return orig;
 }