diff --git a/src/Atlas.cpp b/src/Atlas.cpp
index 23627da..b2c3e25 100644
--- a/src/Atlas.cpp
+++ b/src/Atlas.cpp
@@ -321,7 +321,7 @@ bool Atlas::getGlyphGeometryPair(const unsigned char character,
                                  const FontVariation variation,
                                  GlyphGeometry & a,
                                  GlyphGeometry & b,
-                                 float & mix){
+                                 float & mix) const {
     float smallestDistanceDown = -1 * FLT_MAX;
     float smallestDistanceUp = FLT_MAX;
     int closestIndexDown = 0;
diff --git a/src/Atlas.h b/src/Atlas.h
index b529eca..5f27a87 100644
--- a/src/Atlas.h
+++ b/src/Atlas.h
@@ -98,7 +98,7 @@ class Atlas {
                                   const FontVariation variation,
                                   GlyphGeometry & a,
                                   GlyphGeometry & b,
-                                  float & mix);
+                                  float & mix) const;
 
         //const GlyphGeometry & getGlyphGeometry(GlyphVariationKey key);
         const FontGeometry & getFontGeometry();