diff --git a/src/Makevars.in b/src/Makevars.in
index 548bbc3..0d02d74 100644
--- a/src/Makevars.in
+++ b/src/Makevars.in
@@ -20,8 +20,13 @@ $(STATLIB):
 	    cp ./cargo_vendor_config.toml ./rust/.cargo/config.toml; \
 	fi
 
+# Note: -fno-lto is appended to CFLAGS to override any -flto* injected by R
+# (e.g. the gcc-SAN builder uses -flto=10). The cc crate compiles savvy's
+# unwind_protect_wrapper.c with these CFLAGS and archives the result with
+# plain `ar`, which doesn't record GCC LTO symbols, leaving
+# unwind_protect_impl undefined at dyn.load time.
 	export CC="$(CC)" && \
-	export CFLAGS="$(CFLAGS)" && \
+	export CFLAGS="$(CFLAGS) -fno-lto" && \
 	if [ "$(TARGET)" != "wasm32-unknown-emscripten" ]; then \
 	  @BEFORE_CARGO_BUILD@ cd ./rust && cargo build --jobs 1 --lib --release $(OFFLINE_OPTION); \
 	else \