diff --git a/build/Makefile.in b/build/Makefile.in
index deb20252eee5..d28900e36362 100644
--- a/build/Makefile.in
+++ b/build/Makefile.in
@@ -134,6 +134,26 @@ check::
         else \
           echo "TEST-PASS | build/ | unify produced a universal binary!"; \
         fi
+# try building an x86-64 binary. if that succeeds, try unifying it
+# with an i386 binary
+	-$(HOST_CC) -arch x86_64 $(srcdir)/unify-test.c -o unify-test-x86_64
+	@if test -f ./unify-test-x86_64; then \
+          if ! $(srcdir)/macosx/universal/unify ./unify-test-x86_64 ./unify-test-i386 \
+            ./unify-test-universal-64; then \
+            echo "TEST-UNEXPECTED-FAIL | build/ | unify failed to produce a universal binary with a 64-bit input!"; \
+            false; \
+          fi; \
+          if test ! -f ./unify-test-universal-64; then \
+            echo "TEST-UNEXPECTED-FAIL | build/ | unify failed to produce a universal binary with a 64-bit input!"; \
+            false; \
+          fi; \
+          if ! file -b ./unify-test-universal-64 | head -n1 | grep -q "^Mach-O universal binary"; then \
+            echo "TEST-UNEXPECTED-FAIL | build/ | unify failed to produce a universal binary with a 64-bit input!"; \
+            false; \
+          else \
+            echo "TEST-PASS | build/ | unify produced a universal binary with a 64-bit input!"; \
+          fi \
+        fi
 # try unifying two identical Java class files
 	rm -f unifytesta.class unifytestb.class unifytestc.class
 	cp $(srcdir)/unifytest.class ./unifytesta.class
diff --git a/build/macosx/universal/unify b/build/macosx/universal/unify
index 4730e6cc1bdf..dafd06d1a53f 100755
--- a/build/macosx/universal/unify
+++ b/build/macosx/universal/unify
@@ -1165,6 +1165,12 @@ sub readZipCRCs($) {
 
   package FileAttrCache;
 
+  # from /usr/include/mach-o/loader.h
+  use constant MH_MAGIC    => 0xfeedface;
+  use constant MH_CIGAM    => 0xcefaedfe;
+  use constant MH_MAGIC_64 => 0xfeedfacf;
+  use constant MH_CIGAM_64 => 0xcffaedfe;
+
   use Fcntl(':DEFAULT', ':mode');
 
   # FileAttrCache->new($path)
@@ -1243,8 +1249,10 @@ sub readZipCRCs($) {
     }
 
     # Accept Mach-O fat files or Mach-O thin files of either endianness.
-    if ($magic == 0xfeedface ||
-        $magic == 0xcefaedfe ||
+    if ($magic == MH_MAGIC ||
+        $magic == MH_CIGAM ||
+        $magic == MH_MAGIC_64 ||
+        $magic == MH_CIGAM_64 ||
         $this->isFat()) {
       return 1;
     }