diff --git a/src/cborparser.c b/src/cborparser.c
index 74d91a30..45ae23fb 100644
--- a/src/cborparser.c
+++ b/src/cborparser.c
@@ -1211,7 +1211,7 @@ static CborError iterate_string_chunks(const CborValue *value, char *buffer, siz
             return CborErrorDataTooLarge;
 
         if (*result && *buflen >= newTotal)
-            *result = !!func(buffer + total, (const uint8_t *)ptr, chunkLen);
+            *result = !!func(buffer == NULL ? buffer : buffer + total, (const uint8_t *)ptr, chunkLen);
         else
             *result = false;
 
@@ -1221,7 +1221,7 @@ static CborError iterate_string_chunks(const CborValue *value, char *buffer, siz
     /* is there enough room for the ending NUL byte? */
     if (*result && *buflen > total) {
         uint8_t nul[] = { 0 };
-        *result = !!func(buffer + total, nul, 1);
+        *result = !!func(buffer == NULL ? buffer : buffer + total, nul, 1);
     }
     *buflen = total;
     return _cbor_value_finish_string_iteration(next);