diff --git a/cranelift/tests/moduletests.rs b/cranelift/tests/moduletests.rs
new file mode 100644
index 0000000000..d829aa8e9e
--- /dev/null
+++ b/cranelift/tests/moduletests.rs
@@ -0,0 +1,22 @@
+extern crate cranelift_codegen;
+extern crate cranelift_module;
+extern crate cranelift_simplejit;
+
+use cranelift_codegen::settings::*;
+use cranelift_codegen::ir::*;
+use cranelift_module::*;
+use cranelift_simplejit::*;
+
+#[test]
+fn error_on_incompatible_sig_in_declare_function() {
+    let mut module: Module<SimpleJITBackend> = Module::new(SimpleJITBuilder::new());
+    let mut sig = Signature {
+        params: vec![AbiParam::new(types::I64)],
+        returns: vec![],
+        call_conv: CallConv::SystemV,
+        argument_bytes: None,
+    };
+    module.declare_function("abc", Linkage::Local, &sig).unwrap();
+    sig.params[0] = AbiParam::new(types::I32);
+    module.declare_function("abc", Linkage::Local, &sig).err().unwrap(); // Make sure this is an error
+}
diff --git a/lib/module/src/module.rs b/lib/module/src/module.rs
index e5b7c81883..10008caa51 100644
--- a/lib/module/src/module.rs
+++ b/lib/module/src/module.rs
@@ -159,8 +159,12 @@ impl<B> ModuleFunction<B>
 where
     B: Backend,
 {
-    fn merge(&mut self, linkage: Linkage) {
+    fn merge(&mut self, linkage: Linkage, sig: &ir::Signature) -> Result<(), ModuleError> {
         self.decl.linkage = Linkage::merge(self.decl.linkage, linkage);
+        if &self.decl.signature != sig {
+            return Err(ModuleError::IncompatibleDeclaration(self.decl.name.clone()));
+        }
+        Ok(())
     }
 }
 
@@ -357,7 +361,7 @@ where
             Occupied(entry) => match *entry.get() {
                 FuncOrDataId::Func(id) => {
                     let existing = &mut self.contents.functions[id];
-                    existing.merge(linkage);
+                    existing.merge(linkage, signature)?;
                     self.backend.declare_function(name, existing.decl.linkage);
                     Ok(id)
                 }