diff --git a/patch-delta.c b/patch-delta.c
index 283fb4b759..b5c8594db6 100644
--- a/patch-delta.c
+++ b/patch-delta.c
@@ -40,13 +40,20 @@ void *patch_delta(const void *src_buf, unsigned long src_size,
 		cmd = *data++;
 		if (cmd & 0x80) {
 			unsigned long cp_off = 0, cp_size = 0;
-			if (cmd & 0x01) cp_off = *data++;
-			if (cmd & 0x02) cp_off |= (*data++ << 8);
-			if (cmd & 0x04) cp_off |= (*data++ << 16);
-			if (cmd & 0x08) cp_off |= ((unsigned) *data++ << 24);
-			if (cmd & 0x10) cp_size = *data++;
-			if (cmd & 0x20) cp_size |= (*data++ << 8);
-			if (cmd & 0x40) cp_size |= (*data++ << 16);
+#define PARSE_CP_PARAM(bit, var, shift) do { \
+			if (cmd & (bit)) { \
+				if (data >= top) \
+					goto bad_length; \
+				var |= ((unsigned) *data++ << (shift)); \
+			} } while (0)
+			PARSE_CP_PARAM(0x01, cp_off, 0);
+			PARSE_CP_PARAM(0x02, cp_off, 8);
+			PARSE_CP_PARAM(0x04, cp_off, 16);
+			PARSE_CP_PARAM(0x08, cp_off, 24);
+			PARSE_CP_PARAM(0x10, cp_size, 0);
+			PARSE_CP_PARAM(0x20, cp_size, 8);
+			PARSE_CP_PARAM(0x40, cp_size, 16);
+#undef PARSE_CP_PARAM
 			if (cp_size == 0) cp_size = 0x10000;
 			if (unsigned_add_overflows(cp_off, cp_size) ||
 			    cp_off + cp_size > src_size ||
diff --git a/t/t5303-pack-corruption-resilience.sh b/t/t5303-pack-corruption-resilience.sh
index 41dc947d3f..b68bbeedcc 100755
--- a/t/t5303-pack-corruption-resilience.sh
+++ b/t/t5303-pack-corruption-resilience.sh
@@ -364,7 +364,7 @@ test_expect_success \
 #
 # Note that the literal byte is necessary to get past the uninteresting minimum
 # delta size check.
-test_expect_failure \
+test_expect_success \
     'apply delta with truncated copy parameters' \
     'printf "\5\5\1X\221" > truncated_copy_delta &&
      echo base >base &&