diff --git a/dlls/dplayx/dplay.c b/dlls/dplayx/dplay.c
index daa9f82..75a5261 100644
--- a/dlls/dplayx/dplay.c
+++ b/dlls/dplayx/dplay.c
@@ -4227,11 +4227,21 @@ static HRESULT WINAPI DP_IF_InitializeConnection
 
   TRACE("(%p)->(%p,0x%08x,%u)\n", This, lpConnection, dwFlags, bAnsi );
 
+  if ( lpConnection == NULL )
+  {
+    return DPERR_INVALIDPARAMS;
+  }
+
   if( dwFlags != 0 )
   {
     return DPERR_INVALIDFLAGS;
   }
 
+  if( This->dp2->connectionInitialized != NO_PROVIDER )
+  {
+    return DPERR_ALREADYINITIALIZED;
+  }
+
   /* Find out what the requested SP is and how large this buffer is */
   hr = DPL_EnumAddress( DP_GetSpLpGuidFromCompoundAddress, lpConnection,
                         dwAddrSize, &guidSP );
diff --git a/dlls/dplayx/tests/dplayx.c b/dlls/dplayx/tests/dplayx.c
index cd5e98e..9774624 100644
--- a/dlls/dplayx/tests/dplayx.c
+++ b/dlls/dplayx/tests/dplayx.c
@@ -702,6 +702,53 @@ static void test_EnumConnections()
 
 }
 
+/* InitializeConnection */
+
+static BOOL CALLBACK EnumConnections_cb2( LPCGUID lpguidSP,
+                                          LPVOID lpConnection,
+                                          DWORD dwConnectionSize,
+                                          LPCDPNAME lpName,
+                                          DWORD dwFlags,
+                                          LPVOID lpContext )
+{
+    LPDIRECTPLAY4 pDP = (LPDIRECTPLAY4) lpContext;
+    HRESULT hr;
+
+    /* Incorrect parameters */
+    hr = IDirectPlayX_InitializeConnection( pDP, NULL, 1 );
+    _okHR( DPERR_INVALIDPARAMS, hr );
+    hr = IDirectPlayX_InitializeConnection( pDP, lpConnection, 1 );
+    _okHR( DPERR_INVALIDFLAGS, hr );
+
+    /* Normal operation.
+       We're only interested in ensuring that the TCP/IP provider works */
+
+    if( IsEqualGUID(lpguidSP, &DPSPGUID_TCPIP) )
+    {
+        hr = IDirectPlayX_InitializeConnection( pDP, lpConnection, 0 );
+        todo_wine _okHR( DP_OK, hr );
+        hr = IDirectPlayX_InitializeConnection( pDP, lpConnection, 0 );
+        todo_wine _okHR( DPERR_ALREADYINITIALIZED, hr );
+    }
+
+    return TRUE;
+}
+
+static void test_InitializeConnection()
+{
+
+    LPDIRECTPLAY4 pDP;
+
+    CoCreateInstance( &CLSID_DirectPlay, NULL, CLSCTX_ALL,
+                      &IID_IDirectPlay4A, (LPVOID*) &pDP );
+
+    IDirectPlayX_EnumConnections( pDP, &appGuid, EnumConnections_cb2,
+                                  (LPVOID) pDP, 0 );
+
+    IDirectPlayX_Release( pDP );
+
+}
+
 
 START_TEST(dplayx)
 {
@@ -711,6 +758,7 @@ START_TEST(dplayx)
 
     test_DirectPlayCreate();
     test_EnumConnections();
+    test_InitializeConnection();
 
 
     HeapDestroy( heap );