diff --git a/src/classify/featdefs.cpp b/src/classify/featdefs.cpp
index f1068c12..139e3bfb 100644
--- a/src/classify/featdefs.cpp
+++ b/src/classify/featdefs.cpp
@@ -29,10 +29,10 @@
 /*-----------------------------------------------------------------------------
         Global Data Definitions and Declarations
 -----------------------------------------------------------------------------*/
-const char* kMicroFeatureType = "mf";
-const char* kCNFeatureType = "cn";
-const char* kIntFeatureType = "if";
-const char* kGeoFeatureType = "tb";
+constexpr const char* kMicroFeatureType = "mf";
+constexpr const char* kCNFeatureType = "cn";
+constexpr const char* kIntFeatureType = "if";
+constexpr const char* kGeoFeatureType = "tb";
 
 // Define all of the parameters for the MicroFeature type.
 StartParamDesc(MicroFeatureParams)
diff --git a/src/classify/featdefs.h b/src/classify/featdefs.h
index 6996898c..15228660 100644
--- a/src/classify/featdefs.h
+++ b/src/classify/featdefs.h
@@ -26,10 +26,10 @@
 
 /* Enumerate the different types of features currently defined. */
 #define NUM_FEATURE_TYPES 4
-extern const char* kMicroFeatureType;
-extern const char* kCNFeatureType;
-extern const char* kIntFeatureType;
-extern const char* kGeoFeatureType;
+extern const char* const kMicroFeatureType;
+extern const char* const kCNFeatureType;
+extern const char* const kIntFeatureType;
+extern const char* const kGeoFeatureType;
 
 /* A character is described by multiple sets of extracted features.  Each
   set contains a number of features of a particular type, for example, a