diff --git a/common/JackPortType.cpp b/common/JackPortType.cpp
index 940ef492..86bc876c 100644
--- a/common/JackPortType.cpp
+++ b/common/JackPortType.cpp
@@ -30,7 +30,7 @@ static const JackPortType* gPortTypes[] =
     &gMidiPortType,
 };
 
-jack_port_type_id_t PORT_TYPES_MAX = sizeof(gPortTypes) / sizeof(gPortTypes[0]);
+const jack_port_type_id_t PORT_TYPES_MAX = sizeof(gPortTypes) / sizeof(gPortTypes[0]);
 
 jack_port_type_id_t GetPortTypeId(const char* port_type)
 {
diff --git a/common/JackPortType.h b/common/JackPortType.h
index efb8ca2f..059d955a 100644
--- a/common/JackPortType.h
+++ b/common/JackPortType.h
@@ -27,7 +27,7 @@ Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
 namespace Jack
 {
 
-extern jack_port_type_id_t PORT_TYPES_MAX;
+extern const jack_port_type_id_t PORT_TYPES_MAX;
 
 struct JackPortType
 {