diff --git a/include/sta/ClkInfo.hh b/include/sta/ClkInfo.hh
index 02db3a4d..1844484b 100644
--- a/include/sta/ClkInfo.hh
+++ b/include/sta/ClkInfo.hh
@@ -102,7 +102,7 @@ protected:
 class ClkInfoHash
 {
 public:
-  size_t operator()(const ClkInfo *clk_info);
+  size_t operator()(const ClkInfo *clk_info) const;
 };
 
 class ClkInfoEqual
@@ -110,7 +110,7 @@ class ClkInfoEqual
 public:
   ClkInfoEqual(const StaState *sta);
   bool operator()(const ClkInfo *clk_info1,
-		  const ClkInfo *clk_info2);
+		  const ClkInfo *clk_info2) const;
 
 protected:
   const StaState *sta_;
diff --git a/search/ClkInfo.cc b/search/ClkInfo.cc
index 62d81b41..95f51256 100644
--- a/search/ClkInfo.cc
+++ b/search/ClkInfo.cc
@@ -171,7 +171,7 @@ ClkInfo::refsFilter(const StaState *sta) const
 ////////////////////////////////////////////////////////////////
 
 size_t
-ClkInfoHash::operator()(const ClkInfo *clk_info)
+ClkInfoHash::operator()(const ClkInfo *clk_info) const
 {
   return clk_info->hash();
 }
@@ -185,7 +185,7 @@ ClkInfoEqual::ClkInfoEqual(const StaState *sta) :
 
 bool
 ClkInfoEqual::operator()(const ClkInfo *clk_info1,
-			 const ClkInfo *clk_info2)
+			 const ClkInfo *clk_info2) const
 {
   return clkInfoEqual(clk_info1, clk_info2, sta_);
 }