diff --git a/llvm/_core.cpp b/llvm/_core.cpp
index 7a87c5c..5acfa83 100644
--- a/llvm/_core.cpp
+++ b/llvm/_core.cpp
@@ -280,7 +280,7 @@ _wLLVMLinkModules(PyObject *self, PyObject *args)
 
     PyObject *ret;
     char *errmsg;
-    if (!LLVMLinkModules(dest, src, (LLVMLinkerMode)mode, &errmsg)) {
+    if (LLVMLinkModules(dest, src, (LLVMLinkerMode)mode, &errmsg)) {
         if (errmsg) {
             ret = PyUnicode_FromString(errmsg);
             LLVMDisposeMessage(errmsg);
diff --git a/llvm/core.py b/llvm/core.py
index 3baae95..66d7fbc 100644
--- a/llvm/core.py
+++ b/llvm/core.py
@@ -466,10 +466,12 @@ class Module(llvm.Ownable, llvm.Cacheable):
         check_is_module(other)
         if not preserve:
             other.forget() # remove it from object cache
-        _core.LLVMLinkModules(self.ptr, other.ptr, int(bool(preserve)))
+        result = _core.LLVMLinkModules(self.ptr, other.ptr, int(bool(preserve)))
+        if result is not None:
+            raise llvm.LLVMException(result)
         if not preserve:
-            # Do not try to destroy the other module's llvm::Module*.
-            other._own(llvm.DummyOwner())
+            # Prevent user from using the other module
+            del other.ptr
 
     def get_type_named(self, name):
         """Return a Type object with the given name."""
@@ -1865,8 +1867,9 @@ class Builder(object):
 
     def branch(self, bblk):
         check_is_basic_block(bblk)
-        for instr in self.basic_block.instructions:
-            assert not instr.is_terminator, "BasicBlock can only have one terminator"
+        if __debug__:
+            for instr in self.basic_block.instructions:
+                assert not instr.is_terminator, "BasicBlock can only have one terminator"
         return _make_value(_core.LLVMBuildBr(self.ptr, bblk.ptr))
 
     def cbranch(self, if_value, then_blk, else_blk):