diff --git a/core/src/main/java/hudson/Functions.java b/core/src/main/java/hudson/Functions.java
index ee00f67e75..8c4450c1bb 100644
--- a/core/src/main/java/hudson/Functions.java
+++ b/core/src/main/java/hudson/Functions.java
@@ -2293,12 +2293,17 @@ public class Functions {
             return null;
         }
 
-        StaplerRequest currentRequest = Stapler.getCurrentRequest();
-        currentRequest.getWebApp().getDispatchValidator().allowDispatch(currentRequest, Stapler.getCurrentResponse());
         Icon iconMetadata = IconSet.icons.getIconByClassSpec(iconGuess);
 
+        // `iconGuess` must be class names if it contains a whitespace.
+        //  It may contains extra css classes unrelated to icons.
+        // Filter classes with `icon-` prefix.
+        if (iconMetadata == null && iconGuess.contains(" ")) {
+            iconMetadata = IconSet.icons.getIconByClassSpec(filterIconNameClasses(iconGuess));
+        }
+
         if (iconMetadata == null) {
-            // Icon could be provided as a simple iconFileName e.g. "settings.png"
+            // Icon could be provided as a simple iconFileName e.g. "help.svg"
             iconMetadata = IconSet.icons.getIconByClassSpec(IconSet.toNormalizedIconNameClass(iconGuess) + " icon-md");
         }
 
@@ -2310,6 +2315,12 @@ public class Functions {
         return iconMetadata;
     }
 
+    private static @NonNull String filterIconNameClasses(@NonNull String classNames) {
+        return Arrays.stream(StringUtils.split(classNames, ' '))
+            .filter(className -> className.startsWith("icon-"))
+            .collect(Collectors.joining(" "));
+    }
+
     @Restricted(NoExternalUse.class)
     public static String extractPluginNameFromIconSrc(String iconSrc) {
         if (iconSrc == null) {
diff --git a/core/src/main/java/org/jenkins/ui/icon/Icon.java b/core/src/main/java/org/jenkins/ui/icon/Icon.java
index d4a5e9910c..c639c5cbb7 100644
--- a/core/src/main/java/org/jenkins/ui/icon/Icon.java
+++ b/core/src/main/java/org/jenkins/ui/icon/Icon.java
@@ -68,7 +68,7 @@ public class Icon {
     /**
      * Creates a {@link IconType#CORE core} icon.
      *
-     * @param classSpec The icon class names.
+     * @param classSpec The icon class names. Expected to start with `icon-`.
      * @param style     The icon style.
      */
     public Icon(String classSpec, String style) {
@@ -78,7 +78,7 @@ public class Icon {
     /**
      * Creates a {@link IconType#CORE core} icon.
      *
-     * @param classSpec The icon class names.
+     * @param classSpec The icon class names. Expected to start with `icon-`.
      * @param url       The icon image url.
      * @param style     The icon style.
      */
@@ -96,7 +96,7 @@ public class Icon {
     /**
      * Icon instance.
      *
-     * @param classSpec The icon class specification.
+     * @param classSpec The icon class specification. Expected to start with `icon-`.
      * @param url       The icon image url.
      * @param style     The icon style.
      * @param iconType  The icon type.
@@ -108,7 +108,7 @@ public class Icon {
     /**
      * Creates an icon.
      *
-     * @param classSpec The icon class names.
+     * @param classSpec The icon class names. Expected to start with `icon-`.
      * @param url       The icon image url.
      * @param style     The icon style.
      * @param iconFormat the {@link IconFormat}.
diff --git a/core/src/test/java/hudson/FunctionsTest.java b/core/src/test/java/hudson/FunctionsTest.java
index 85ae2ca704..fff8391321 100644
--- a/core/src/test/java/hudson/FunctionsTest.java
+++ b/core/src/test/java/hudson/FunctionsTest.java
@@ -28,6 +28,8 @@ import static org.hamcrest.MatcherAssert.assertThat;
 import static org.hamcrest.Matchers.emptyString;
 import static org.hamcrest.Matchers.equalTo;
 import static org.hamcrest.Matchers.is;
+import static org.hamcrest.Matchers.not;
+import static org.hamcrest.Matchers.nullValue;
 import static org.junit.Assert.assertEquals;
 import static org.junit.Assert.assertNull;
 import static org.junit.Assert.assertTrue;
@@ -639,4 +641,39 @@ public class FunctionsTest {
             return this;
         }
     }
+
+    @Test
+    public void tryGetIcon_shouldReturnNullForNull() throws Exception {
+        assertThat(Functions.tryGetIcon(null), is(nullValue()));
+    }
+
+    @Test
+    public void tryGetIcon_shouldReturnNullForSymbol() throws Exception {
+        assertThat(Functions.tryGetIcon("symbol-search"), is(nullValue()));
+    }
+
+    @Test
+    public void tryGetIcon_shouldReturnMetadataForExactSpec() throws Exception {
+        assertThat(Functions.tryGetIcon("icon-help icon-sm"), is(not(nullValue())));
+    }
+
+    @Test
+    public void tryGetIcon_shouldReturnMetadataForExtraSpec() throws Exception {
+        assertThat(Functions.tryGetIcon("icon-help icon-sm extra-class"), is(not(nullValue())));
+    }
+
+    @Test
+    public void tryGetIcon_shouldReturnMetadataForFilename() throws Exception {
+        assertThat(Functions.tryGetIcon("help.svg"), is(not(nullValue())));
+    }
+
+    @Test
+    public void tryGetIcon_shouldReturnMetadataForUrl() throws Exception {
+        assertThat(Functions.tryGetIcon("48x48/green.gif"), is(not(nullValue())));
+    }
+
+    @Test
+    public void tryGetIcon_shouldReturnNullForUnknown() throws Exception {
+        assertThat(Functions.tryGetIcon("icon-nosuchicon"), is(nullValue()));
+    }
 }