diff --git a/formats/format_wav_gsm.c b/formats/format_wav_gsm.c
index ea0f95e22e4c947ffb102bec34e70939689937fa..3a1dfdfb349fcecbae84f1bad3afbe8f679984cb 100644
--- a/formats/format_wav_gsm.c
+++ b/formats/format_wav_gsm.c
@@ -336,7 +336,7 @@ static int write_header(FILE *f)
 		return -1;
 	}
 	/* 32: Subchunk 1: Block align */
-	if (fwrite(&block_align, 1, 2, f) != 4) {
+	if (fwrite(&block_align, 1, 4, f) != 4) {
 		ast_log(LOG_WARNING, "Unable to write header\n");
 		return -1;
 	}