diff --git a/pngwutil.c b/pngwutil.c
index 8f669374c..755ccef39 100644
--- a/pngwutil.c
+++ b/pngwutil.c
@@ -1745,9 +1745,13 @@ png_write_iTXt(png_structrp png_ptr, int compression, png_const_charp key,
    {
       if (comp.input_len > PNG_UINT_31_MAX-prefix_len)
          png_error(png_ptr, "iTXt: uncompressed text too long");
-      png_write_chunk_header(png_ptr, png_iTXt, strlen(text) + prefix_len);
+
+      /* So the string will fit in a chunk: */
+      comp.output_len = (png_uint_32)/*SAFE*/comp.input_len;
    }
 
+   png_write_chunk_header(png_ptr, png_iTXt, comp.output_len + prefix_len);
+
    png_write_chunk_data(png_ptr, new_key, key_len);
 
    png_write_chunk_data(png_ptr, (png_const_bytep)lang, lang_len);