diff --git a/src/backend_x64/emit_x64.cpp b/src/backend_x64/emit_x64.cpp
index f2afbc1a..cc40f810 100644
--- a/src/backend_x64/emit_x64.cpp
+++ b/src/backend_x64/emit_x64.cpp
@@ -99,7 +99,7 @@ EmitX64::BlockDescriptor EmitX64::Emit(IR::Block& block) {
     return basic_blocks[descriptor];
 }
 
-boost::optional<EmitX64::BlockDescriptor> EmitX64::GetBasicBlock(IR::LocationDescriptor descriptor) {
+boost::optional<EmitX64::BlockDescriptor> EmitX64::GetBasicBlock(IR::LocationDescriptor descriptor) const {
     auto iter = basic_blocks.find(descriptor);
     if (iter == basic_blocks.end())
         return boost::none;
diff --git a/src/backend_x64/emit_x64.h b/src/backend_x64/emit_x64.h
index f4585a6d..1308de54 100644
--- a/src/backend_x64/emit_x64.h
+++ b/src/backend_x64/emit_x64.h
@@ -47,7 +47,7 @@ public:
     BlockDescriptor Emit(IR::Block& ir);
 
     /// Looks up an emitted host block in the cache.
-    boost::optional<BlockDescriptor> GetBasicBlock(IR::LocationDescriptor descriptor);
+    boost::optional<BlockDescriptor> GetBasicBlock(IR::LocationDescriptor descriptor) const;
 
     /// Empties the cache.
     void ClearCache();