diff --git a/cdcacm.h b/cdcacm.h
index 8372f56b..2a38524d 100644
--- a/cdcacm.h
+++ b/cdcacm.h
@@ -204,7 +204,7 @@ public:
         uint8_t Poll();
 
         bool available(void) {
-
+                return false;
         };
 
         virtual uint8_t GetAddress() {
diff --git a/cdcftdi.h b/cdcftdi.h
index 5a133241..b7312526 100644
--- a/cdcftdi.h
+++ b/cdcftdi.h
@@ -80,9 +80,11 @@ class FTDIAsyncOper {
 public:
 
         virtual uint8_t OnInit(FTDI *pftdi) {
+                return 0;
         };
 
         virtual uint8_t OnRelease(FTDI *pftdi) {
+                return 0;
         };
 };
 
diff --git a/hid.h b/hid.h
index 14507183..72942ebc 100644
--- a/hid.h
+++ b/hid.h
@@ -156,7 +156,9 @@ protected:
         void PrintEndpointDescriptor(const USB_ENDPOINT_DESCRIPTOR* ep_ptr);
         void PrintHidDescriptor(const USB_HID_DESCRIPTOR *pDesc);
 
-        virtual HIDReportParser* GetReportParser(uint8_t id) {};
+        virtual HIDReportParser* GetReportParser(uint8_t id) {
+                return NULL;
+        };
 
 public:
 
@@ -167,7 +169,9 @@ public:
                 return pUsb;
         };
 
-        virtual bool SetReportParser(uint8_t id, HIDReportParser *prs) {};
+        virtual bool SetReportParser(uint8_t id, HIDReportParser *prs) {
+                return false;
+        };
 
         uint8_t SetProtocol(uint8_t iface, uint8_t protocol);
         uint8_t GetProtocol(uint8_t iface, uint8_t* dataptr);