diff --git a/checksum.c b/checksum.c
index 66e80896..2b2277c2 100644
--- a/checksum.c
+++ b/checksum.c
@@ -373,25 +373,27 @@ void get_checksum2(char *buf, int32 len, char *sum)
 			len1 = len;
 		}
 
-		memcpy(buf1, buf, len);
-		if (checksum_seed) {
-			SIVAL(buf1,len,checksum_seed);
-			len += 4;
-		}
+		if (buf1) {
+			memcpy(buf1, buf, len);
+			if (checksum_seed) {
+				SIVAL(buf1,len,checksum_seed);
+				len += 4;
+			}
 
-		for (i = 0; i + CSUM_CHUNK <= len; i += CSUM_CHUNK)
-			mdfour_update(&m, (uchar *)(buf1+i), CSUM_CHUNK);
+			for (i = 0; i + CSUM_CHUNK <= len; i += CSUM_CHUNK)
+				mdfour_update(&m, (uchar *)(buf1+i), CSUM_CHUNK);
 
-		/*
-		 * Prior to version 27 an incorrect MD4 checksum was computed
-		 * by failing to call mdfour_tail() for block sizes that
-		 * are multiples of 64.  This is fixed by calling mdfour_update()
-		 * even when there are no more bytes.
-		 */
-		if (len - i > 0 || xfer_sum_nni->num > CSUM_MD4_BUSTED)
-			mdfour_update(&m, (uchar *)(buf1+i), len-i);
+			/*
+			 * Prior to version 27 an incorrect MD4 checksum was computed
+			 * by failing to call mdfour_tail() for block sizes that
+			 * are multiples of 64.  This is fixed by calling mdfour_update()
+			 * even when there are no more bytes.
+			 */
+			if (len - i > 0 || xfer_sum_nni->num > CSUM_MD4_BUSTED)
+				mdfour_update(&m, (uchar *)(buf1+i), len-i);
 
-		mdfour_result(&m, (uchar *)sum);
+			mdfour_result(&m, (uchar *)sum);
+		}
 		break;
 	  }
 	  default: /* paranoia to prevent missing case values */