diff --git a/src/fortran/matio_t.inc.in b/src/fortran/matio_t.inc.in
index 48b6091c..aab2a7f8 100644
--- a/src/fortran/matio_t.inc.in
+++ b/src/fortran/matio_t.inc.in
@@ -90,6 +90,7 @@ END TYPE matvar_t
     INTEGER,PARAMETER :: MAT_C_INT64      = 14
     INTEGER,PARAMETER :: MAT_C_UINT64     = 15
     INTEGER,PARAMETER :: MAT_C_FUNCTION   = 16
+    INTEGER,PARAMETER :: MAT_C_OPAQUE     = 17
 
     INTEGER,PARAMETER :: COMPRESSION_NONE = 0
     INTEGER,PARAMETER :: COMPRESSION_ZLIB = 1