diff --git a/src/main/kotlin/nl/avisi/structurizr/site/generatr/StringUtilities.kt b/src/main/kotlin/nl/avisi/structurizr/site/generatr/StringUtilities.kt
index 1afba68e..6599720d 100644
--- a/src/main/kotlin/nl/avisi/structurizr/site/generatr/StringUtilities.kt
+++ b/src/main/kotlin/nl/avisi/structurizr/site/generatr/StringUtilities.kt
@@ -1,3 +1,21 @@
 package nl.avisi.structurizr.site.generatr
 
-fun String.normalize(): String = lowercase().replace("\\s+".toRegex(), "-")
+// based on https://stackoverflow.com/questions/1976007/what-characters-are-forbidden-in-windows-and-linux-directory-names
+private const val reservedChars = "|\\?*<\":>+[]/'"
+private val reservedNames = setOf(
+    "CON", "PRN", "AUX", "NUL",
+    "COM1", "COM2", "COM3", "COM4", "COM5", "COM6", "COM7", "COM8", "COM9",
+    "LPT1", "LPT2", "LPT3", "LPT4", "LPT5", "LPT6", "LPT7", "LPT8", "LPT9"
+)
+
+fun String.normalize(): String =
+    lowercase()
+        .replace("\\s+".toRegex(), "-")
+        .filterNot { reservedChars.contains(it) }
+        .trim()
+        .let {
+            if (reservedNames.contains(it.uppercase()))
+                "${it}-"
+            else
+                it
+        }
diff --git a/src/test/kotlin/nl/avisi/structurizr/site/generatr/site/StringUtilitiesTest.kt b/src/test/kotlin/nl/avisi/structurizr/site/generatr/site/StringUtilitiesTest.kt
new file mode 100644
index 00000000..0b959d21
--- /dev/null
+++ b/src/test/kotlin/nl/avisi/structurizr/site/generatr/site/StringUtilitiesTest.kt
@@ -0,0 +1,23 @@
+package nl.avisi.structurizr.site.generatr.site
+
+import assertk.assertThat
+import assertk.assertions.isEqualTo
+import nl.avisi.structurizr.site.generatr.normalize
+import org.junit.jupiter.api.DynamicTest
+import org.junit.jupiter.api.TestFactory
+
+class StringUtilitiesTest {
+
+    @TestFactory
+    fun `normalize strips invalid chars`() = listOf(
+        listOf("doc", "doc"),
+        listOf("d c", "d-c"),
+        listOf("doc:", "doc"),
+        listOf(" doc ", "-doc-"),
+        listOf("aux", "aux-"),
+    ).map { (actual, expected) ->
+        DynamicTest.dynamicTest("normalize replaces $actual with $expected") {
+            assertThat(actual.normalize()).isEqualTo(expected)
+        }
+    }
+}
diff --git a/src/test/kotlin/nl/avisi/structurizr/site/generatr/site/StructurizrUtilitiesTest.kt b/src/test/kotlin/nl/avisi/structurizr/site/generatr/site/StructurizrUtilitiesTest.kt
index 7ebb8900..8657d901 100644
--- a/src/test/kotlin/nl/avisi/structurizr/site/generatr/site/StructurizrUtilitiesTest.kt
+++ b/src/test/kotlin/nl/avisi/structurizr/site/generatr/site/StructurizrUtilitiesTest.kt
@@ -1,15 +1,16 @@
 package nl.avisi.structurizr.site.generatr.site
 
+import assertk.assertThat
+import assertk.assertions.isEqualTo
 import com.structurizr.Workspace
 import nl.avisi.structurizr.site.generatr.includedProperties
 import kotlin.test.Test
-import kotlin.test.assertEquals
 
 class StructurizrUtilitiesTest {
 
-    protected val svgFactory = { _: String, _: String -> "" }
+    private val svgFactory = { _: String, _: String -> "" }
 
-    protected fun generatorContext(
+    private fun generatorContext(
         workspaceName: String = "Workspace name",
         branches: List<String> = listOf("main"),
         currentBranch: String = "main",
@@ -27,8 +28,8 @@ class StructurizrUtilitiesTest {
 
         val includedProperties = element.includedProperties
 
-        assertEquals(3, includedProperties.size)
-        assertEquals(includedProperties.keys, setOf("structurizrnotquite.key", "generatrbut.not.key", "other.key"))
+        assertThat(includedProperties.size).isEqualTo(3)
+        assertThat(includedProperties.keys).isEqualTo(setOf("structurizrnotquite.key", "generatrbut.not.key", "other.key"))
     }
 
     @Test
@@ -39,15 +40,15 @@ class StructurizrUtilitiesTest {
 
         val includedProperties = element.includedProperties
 
-        assertEquals(2, includedProperties.size)
-        assertEquals("key1", includedProperties.keys.first())
-        assertEquals("key2", includedProperties.keys.last())
+        assertThat(includedProperties.size).isEqualTo(2)
+        assertThat(includedProperties.keys.first()).isEqualTo("key1")
+        assertThat(includedProperties.keys.last()).isEqualTo("key2")
     }
 
     @Test
     fun `includedProperties returns empty map when no properties are set`() {
         val element = generatorContext().workspace.model.addSoftwareSystem("System 1")
         val includedProperties = element.includedProperties
-        assertEquals(0, includedProperties.size)
+        assertThat(includedProperties.size).isEqualTo(0)
     }
 }