diff --git a/include/freetype/ftglyph.h b/include/freetype/ftglyph.h
index 67a698f55..e1acae7b8 100644
--- a/include/freetype/ftglyph.h
+++ b/include/freetype/ftglyph.h
@@ -483,7 +483,7 @@ FT_BEGIN_HEADER
   /*    The result is undefined if either `a' or `b' is zero.              */
   /*                                                                       */
   FT_EXPORT( void )
-  FT_Matrix_Multiply( FT_Matrix*  a,
+  FT_Matrix_Multiply( const FT_Matrix*  a,
                       FT_Matrix*  b );
 
 
diff --git a/src/base/ftglyph.c b/src/base/ftglyph.c
index 9f3d6fd4b..dbd31ea82 100644
--- a/src/base/ftglyph.c
+++ b/src/base/ftglyph.c
@@ -56,7 +56,7 @@
   /* documentation is in ftglyph.h */
 
   FT_EXPORT_DEF( void )
-  FT_Matrix_Multiply( FT_Matrix*  a,
+  FT_Matrix_Multiply( const FT_Matrix*  a,
                       FT_Matrix*  b )
   {
     FT_Fixed  xx, xy, yx, yy;