diff --git a/internal/ceres/parameter_block.h b/internal/ceres/parameter_block.h
index 3a709f4..b1e8d93 100644
--- a/internal/ceres/parameter_block.h
+++ b/internal/ceres/parameter_block.h
@@ -73,7 +73,7 @@
 
   // Create a parameter block with the user state, size, and index specified.
   // The size is the size of the parameter block and the index is the position
-  // if the parameter block inside a Program (if any).
+  // of the parameter block inside a Program (if any).
   ParameterBlock(double* user_state, int size, int index) {
     Init(user_state, size, index, NULL);
   }