diff --git a/srcpkgs/flatbuffers/patches/init-var.patch b/srcpkgs/flatbuffers/patches/init-var.patch
new file mode 100644
index 00000000000..eb7de9655a2
--- /dev/null
+++ b/srcpkgs/flatbuffers/patches/init-var.patch
@@ -0,0 +1,13 @@
+Index: flatbuffers-1.12.0/tests/test_builder.h
+===================================================================
+--- flatbuffers-1.12.0.orig/tests/test_builder.h
++++ flatbuffers-1.12.0/tests/test_builder.h
+@@ -63,7 +63,7 @@ template<class Builder>
+ void builder_move_assign_after_releaseraw_test(Builder b1) {
+   auto root_offset1 = populate1(b1);
+   b1.Finish(root_offset1);
+-  size_t size, offset;
++  size_t size = 0, offset;
+   std::shared_ptr<uint8_t> raw(
+       b1.ReleaseRaw(size, offset), [size](uint8_t *ptr) {
+         flatbuffers::DefaultAllocator::dealloc(ptr, size);