diff --git a/source/opt/constants.cpp b/source/opt/constants.cpp
index 0887ec2..6057356 100644
--- a/source/opt/constants.cpp
+++ b/source/opt/constants.cpp
@@ -156,7 +156,7 @@
 }
 
 std::vector<const Constant*> ConstantManager::GetOperandConstants(
-    Instruction* inst) const {
+    const Instruction* inst) const {
   std::vector<const Constant*> constants;
   for (uint32_t i = 0; i < inst->NumInOperands(); i++) {
     const Operand* operand = &inst->GetInOperand(i);
diff --git a/source/opt/constants.h b/source/opt/constants.h
index 5f2fdc7..9518b5b 100644
--- a/source/opt/constants.h
+++ b/source/opt/constants.h
@@ -597,7 +597,8 @@
 
   // Returns a vector of constants representing each in operand. If an operand
   // is not constant its entry is nullptr.
-  std::vector<const Constant*> GetOperandConstants(Instruction* inst) const;
+  std::vector<const Constant*> GetOperandConstants(
+      const Instruction* inst) const;
 
   // Records a mapping between |inst| and the constant value generated by it.
   // It returns true if a new Constant was successfully mapped, false if |inst|