diff --git a/nac3core/build.rs b/nac3core/build.rs
index 59dd26fa..7393ca9b 100644
--- a/nac3core/build.rs
+++ b/nac3core/build.rs
@@ -31,6 +31,7 @@ fn compile_irrt(irrt_dir: &Path, out_dir: &Path) {
         "-S",
         "-Wall",
         "-Wextra",
+        "-Werror=return-type",
         "-I",
         irrt_dir.to_str().unwrap(),
         "-o",
@@ -96,6 +97,7 @@ fn compile_irrt_test(irrt_dir: &Path, out_dir: &Path) {
         "-O0",
         "-Wall",
         "-Wextra",
+        "-Werror=return-type",
         "-lm", // for `tgamma()`, `lgamma()`
         "-o",
         exe_path.to_str().unwrap(),