diff --git a/src/Libraries/NRefactory/ICSharpCode.NRefactory.CSharp.Refactoring/CodeActions/AddOptionalParameterToInvocationAction.cs b/src/Libraries/NRefactory/ICSharpCode.NRefactory.CSharp.Refactoring/CodeActions/AddOptionalParameterToInvocationAction.cs
index 49157a040e..5c1ef53474 100644
--- a/src/Libraries/NRefactory/ICSharpCode.NRefactory.CSharp.Refactoring/CodeActions/AddOptionalParameterToInvocationAction.cs
+++ b/src/Libraries/NRefactory/ICSharpCode.NRefactory.CSharp.Refactoring/CodeActions/AddOptionalParameterToInvocationAction.cs
@@ -54,8 +54,10 @@ namespace ICSharpCode.NRefactory.CSharp.Refactoring
 			bool[] parameterIsSpecified = new bool[method.Parameters.Count];
 			var argumentToParameterMap = resolveResult.GetArgumentToParameterMap();
 			if (argumentToParameterMap != null) {
-				foreach (int paramIndex in argumentToParameterMap)
-					parameterIsSpecified[paramIndex] = true;
+				foreach (int paramIndex in argumentToParameterMap) {
+					if (paramIndex >= 0)
+						parameterIsSpecified[paramIndex] = true;
+				}
 			} else {
 				for (int i = 0; i < Math.Min(resolveResult.Arguments.Count, parameterIsSpecified.Length); i++) {
 					parameterIsSpecified[i] = true;
diff --git a/src/Main/SharpDevelop/Parser/AssemblyParserService.cs b/src/Main/SharpDevelop/Parser/AssemblyParserService.cs
index 0c864e2e7f..1b2959abb5 100644
--- a/src/Main/SharpDevelop/Parser/AssemblyParserService.cs
+++ b/src/Main/SharpDevelop/Parser/AssemblyParserService.cs
@@ -391,7 +391,7 @@ namespace ICSharpCode.SharpDevelop.Parser
 				return GetAssemblyModel(fileName, includeInternalMembers);
 			} catch (BadImageFormatException) {
 				SD.MessageService.ShowWarningFormatted("${res:ICSharpCode.SharpDevelop.Dom.AssemblyInvalid}", Path.GetFileName(fileName));
-			} catch (FileNotFoundException) {
+			} catch (IOException) {
 				SD.MessageService.ShowWarningFormatted("${res:ICSharpCode.SharpDevelop.Dom.AssemblyNotAccessible}", fileName);
 			}