diff --git a/base/src/main/java/proguard/UpToDateChecker.java b/base/src/main/java/proguard/UpToDateChecker.java
index f89cf0da..69b2a512 100644
--- a/base/src/main/java/proguard/UpToDateChecker.java
+++ b/base/src/main/java/proguard/UpToDateChecker.java
@@ -136,7 +136,11 @@ public void updateInputModificationTime(URL url)
                 {
                     updateModificationTime(new File(url.toURI()), false);
                 }
-                catch (URISyntaxException ignore) {}
+                catch (URISyntaxException exception)
+                {
+                    // The input couldn't be checked, so conservatively consider the output outdated.
+                    throw new IllegalStateException("Can't check the input modification time", exception);
+                }
             }
         }
 
diff --git a/base/src/test/kotlin/proguard/UpToDateCheckerTest.kt b/base/src/test/kotlin/proguard/UpToDateCheckerTest.kt
new file mode 100644
index 00000000..a5f06c54
--- /dev/null
+++ b/base/src/test/kotlin/proguard/UpToDateCheckerTest.kt
@@ -0,0 +1,37 @@
+package proguard
+
+import io.kotest.assertions.throwables.shouldNotThrowAny
+import io.kotest.core.spec.style.FreeSpec
+import java.io.File
+import java.net.URL
+import java.nio.file.Files
+
+class UpToDateCheckerTest : FreeSpec({
+    "An invalid file URI does not hide an outdated obfuscation dictionary" {
+        val directory = Files.createTempDirectory("proguard-up-to-date-test").toFile()
+
+        try {
+            val output = File(directory, "output.jar").apply {
+                createNewFile()
+                setLastModified(1_000L)
+            }
+            val dictionary = File(directory, "dictionary with spaces.txt").apply {
+                createNewFile()
+                setLastModified(2_000L)
+            }
+
+            val configuration = Configuration().apply {
+                programJars = ClassPath().apply {
+                    add(ClassPathEntry(output, true))
+                }
+                obfuscationDictionary = URL(dictionary.toURI().toString().replace("%20", " "))
+            }
+
+            shouldNotThrowAny {
+                UpToDateChecker(configuration).check()
+            }
+        } finally {
+            directory.deleteRecursively()
+        }
+    }
+})