diff --git a/src/Atlas.cpp b/src/Atlas.cpp
index b2c3e25..c6ffaf8 100644
--- a/src/Atlas.cpp
+++ b/src/Atlas.cpp
@@ -358,8 +358,8 @@ bool Atlas::getGlyphGeometryPair(const unsigned char character,
     const auto & closestVariationDown = variationSteps[closestIndexDown];
     GlyphVariationKey glyphKeyUp = {character, closestVariationUp};
     GlyphVariationKey glyphKeyDown = {character, closestVariationDown};
-    int index_a = glyphGeometryMap[glyphKeyDown];
-    int index_b = glyphGeometryMap[glyphKeyUp];
+    int index_a = glyphGeometryMap.at(glyphKeyDown);
+    int index_b = glyphGeometryMap.at(glyphKeyUp);
     a = glyphGeometries[index_a];
     b = glyphGeometries[index_b];