diff --git a/src/Serilog/LoggerConfigurationFullNetFxExtensions.cs b/src/Serilog/LoggerConfigurationFullNetFxExtensions.cs
index 5e28936c6..602eedffa 100644
--- a/src/Serilog/LoggerConfigurationFullNetFxExtensions.cs
+++ b/src/Serilog/LoggerConfigurationFullNetFxExtensions.cs
@@ -294,6 +294,8 @@ public static LoggerConfiguration AppSettings(
             this LoggerSettingsConfiguration settingConfiguration, string settingPrefix = "serilog")
         {
             if (settingConfiguration == null) throw new ArgumentNullException(nameof(settingConfiguration));
+            if (settingPrefix == null) throw new ArgumentNullException(nameof(settingPrefix));
+            if (settingPrefix.Contains(":")) throw new ArgumentException("Custom setting prefixes cannot contain the colon (:) character.");
             return settingConfiguration.Settings(new AppSettingsSettings(settingPrefix));
         }
 #endif
diff --git a/src/Serilog/Sinks/PeriodicBatching/PortableTimer.cs b/src/Serilog/Sinks/PeriodicBatching/PortableTimer.cs
index ce403b865..f54de2e75 100644
--- a/src/Serilog/Sinks/PeriodicBatching/PortableTimer.cs
+++ b/src/Serilog/Sinks/PeriodicBatching/PortableTimer.cs
@@ -55,7 +55,7 @@ public async void Start(TimeSpan interval)
                 // current API, which allows the tick handler to reenter and set the next interval.
 
                 if (_state == PortableTimerState.Waiting)
-                    throw new InvalidOperationException("The timer is already set");
+                    throw new InvalidOperationException("The timer is already set.");
 
                 if (_cancel.IsCancellationRequested) return;
 
diff --git a/test/Serilog.Tests/Settings/AppSettingsTests.cs b/test/Serilog.Tests/Settings/AppSettingsTests.cs
index 318dfa107..2e14c8664 100644
--- a/test/Serilog.Tests/Settings/AppSettingsTests.cs
+++ b/test/Serilog.Tests/Settings/AppSettingsTests.cs
@@ -1,4 +1,5 @@
 #if !DNXCORE50
+using System;
 using System.Configuration;
 using Xunit;
 using Serilog.Events;
@@ -30,8 +31,8 @@ public void EnvironmentVariableExpansionIsApplied()
         [Fact]
         public void CanUseCustomPrefixToConfigureSettings()
         {
-            const string prefix1 = "custom1:serilog";
-            const string prefix2 = "custom2:serilog";
+            const string prefix1 = "custom1-serilog";
+            const string prefix2 = "custom2-serilog";
 
             // Make sure we have the expected keys in the App.config
             Assert.Equal("Warning", ConfigurationManager.AppSettings[prefix1 + ":minimum-level"]);
@@ -51,6 +52,13 @@ public void CanUseCustomPrefixToConfigureSettings()
             Assert.False(log2.IsEnabled(LogEventLevel.Warning));
             Assert.True(log2.IsEnabled(LogEventLevel.Error));
         }
+
+        [Fact]
+        public void CustomPrefixCannotContainColon()
+        {
+            Assert.Throws<ArgumentException>(() => 
+                new LoggerConfiguration().ReadFrom.AppSettings("custom1:serilog"));
+        }
     }
 }
 #endif
diff --git a/test/Serilog.Tests/app.config b/test/Serilog.Tests/app.config
index 346514b7e..a864cba68 100644
--- a/test/Serilog.Tests/app.config
+++ b/test/Serilog.Tests/app.config
@@ -2,7 +2,7 @@
 <configuration>
   <appSettings>
     <add key="serilog:enrich:with-property:Path" value="%PATH%" />
-    <add key="custom1:serilog:minimum-level" value="Warning" />
-    <add key="custom2:serilog:minimum-level" value="Error" />
+    <add key="custom1-serilog:minimum-level" value="Warning" />
+    <add key="custom2-serilog:minimum-level" value="Error" />
   </appSettings>
 </configuration>
\ No newline at end of file