diff --git a/utf8proc.c b/utf8proc.c
index 6752a55..6d6871d 100644
--- a/utf8proc.c
+++ b/utf8proc.c
@@ -365,7 +365,7 @@ static utf8proc_ssize_t seqindex_write_char_decomposed(utf8proc_uint16_t seqinde
   for (; len >= 0; entry++, len--) {
     utf8proc_int32_t entry_cp = seqindex_decode_entry(&entry);
 
-    written += utf8proc_decompose_char(entry_cp, dst+written,
+    written += utf8proc_decompose_char(entry_cp, dst ? dst+written : dst,
       (bufsize > written) ? (bufsize - written) : 0, options,
     last_boundclass);
     if (written < 0) return UTF8PROC_ERROR_OVERFLOW;
@@ -551,7 +551,7 @@ UTF8PROC_DLLEXPORT utf8proc_ssize_t utf8proc_decompose_custom(
         uc = custom_func(uc, custom_data);   /* user-specified custom mapping */
       }
       decomp_result = utf8proc_decompose_char(
-        uc, buffer + wpos, (bufsize > wpos) ? (bufsize - wpos) : 0, options,
+        uc, buffer ? buffer+wpos : buffer, (bufsize > wpos) ? (bufsize - wpos) : 0, options,
         &boundclass
       );
       if (decomp_result < 0) return decomp_result;