diff --git a/lib/compat/externalcommandlistener.cpp b/lib/compat/externalcommandlistener.cpp
index aac24ecc5..dcd57eb3f 100644
--- a/lib/compat/externalcommandlistener.cpp
+++ b/lib/compat/externalcommandlistener.cpp
@@ -144,7 +144,7 @@ void ExternalCommandListener::CommandPipeThread(const String& commandPath)
 					ExternalCommandProcessor::Execute(command);
 				} catch (const std::exception& ex) {
 					Log(LogWarning, "ExternalCommandListener")
-					    << "External command failed." << DiagnosticInformation(ex);
+					    << "External command failed: " << DiagnosticInformation(ex);
 				}
 			}
 		}
diff --git a/lib/icinga/comment.cpp b/lib/icinga/comment.cpp
index eeb7b2b60..fd98ae120 100644
--- a/lib/icinga/comment.cpp
+++ b/lib/icinga/comment.cpp
@@ -195,6 +195,9 @@ String Comment::AddComment(const Checkable::Ptr& checkable, CommentType entryTyp
 
 	Comment::Ptr comment = Comment::GetByName(fullName);
 
+	if (!comment)
+		BOOST_THROW_EXCEPTION(std::runtime_error("Could not create comment."));
+
 	Log(LogNotice, "Comment")
 	    << "Added comment '" << comment->GetName() << "'.";
 
diff --git a/lib/icinga/downtime.cpp b/lib/icinga/downtime.cpp
index eaa2e9ba2..66b6a635e 100644
--- a/lib/icinga/downtime.cpp
+++ b/lib/icinga/downtime.cpp
@@ -251,12 +251,14 @@ String Downtime::AddDowntime(const Checkable::Ptr& checkable, const String& auth
 
 	Downtime::Ptr downtime = Downtime::GetByName(fullName);
 
+	if (!downtime)
+		BOOST_THROW_EXCEPTION(std::runtime_error("Could not create downtime."));
+
 	Log(LogNotice, "Downtime")
 	    << "Added downtime '" << downtime->GetName()
 	    << "' between '" << Utility::FormatDateTime("%Y-%m-%d %H:%M:%S", startTime)
 	    << "' and '" << Utility::FormatDateTime("%Y-%m-%d %H:%M:%S", endTime) << "'.";
 
-
 	return fullName;
 }
 
diff --git a/lib/icinga/externalcommandprocessor.cpp b/lib/icinga/externalcommandprocessor.cpp
index 6d1711d08..c9c921c41 100644
--- a/lib/icinga/externalcommandprocessor.cpp
+++ b/lib/icinga/externalcommandprocessor.cpp
@@ -1210,7 +1210,7 @@ void ExternalCommandProcessor::AddHostComment(double, const std::vector<String>&
 		BOOST_THROW_EXCEPTION(std::invalid_argument("Cannot add host comment for non-existent host '" + arguments[0] + "'"));
 
 	if (arguments[2].IsEmpty() || arguments[3].IsEmpty())
-		BOOST_THROW_EXCEPTION(std::invalid_argument("Author and comment may not be empty"));
+		BOOST_THROW_EXCEPTION(std::invalid_argument("Author and comment must not be empty"));
 
 	Log(LogNotice, "ExternalCommandProcessor")
 	    << "Creating comment for host " << host->GetName();
@@ -1234,7 +1234,7 @@ void ExternalCommandProcessor::AddSvcComment(double, const std::vector<String>&
 		BOOST_THROW_EXCEPTION(std::invalid_argument("Cannot add service comment for non-existent service '" + arguments[1] + "' on host '" + arguments[0] + "'"));
 
 	if (arguments[3].IsEmpty() || arguments[4].IsEmpty())
-		BOOST_THROW_EXCEPTION(std::invalid_argument("Author and comment may not be empty"));
+		BOOST_THROW_EXCEPTION(std::invalid_argument("Author and comment must not be empty"));
 
 	Log(LogNotice, "ExternalCommandProcessor")
 	    << "Creating comment for service " << service->GetName();