diff --git a/Sources/LocheckLogic/Validators/parseAndValidateXCStrings.swift b/Sources/LocheckLogic/Validators/parseAndValidateXCStrings.swift
index 212eb8d..8a21066 100644
--- a/Sources/LocheckLogic/Validators/parseAndValidateXCStrings.swift
+++ b/Sources/LocheckLogic/Validators/parseAndValidateXCStrings.swift
@@ -83,10 +83,22 @@ func validateStrings(
             path: baseString.path,
             lineNumber: baseString.line)
     }
+        
+    let baseStringsKeys = Set(baseStrings.map(\.key))
+        
+    var missingInBaseKeys: Set<String> = []
+    for translationString in translationStrings where !baseStringsKeys.contains(translationString.key) {
+        problemReporter.report(
+            KeyMissingFromBase(
+                key: translationString.key),
+            path: translationString.path,
+            lineNumber: translationString.line)
+        missingInBaseKeys.insert(translationString.key)
+    }
 
     // MARK: Validate arguments
 
-    for translationString in translationStrings {
+    for translationString in translationStrings where !missingInBaseKeys.contains(translationString.key) {
         let baseArgumentPositions = Set(translationString.base.arguments.map(\.position))
         let translationArgumentPositions = Set(translationString.translation.arguments.map(\.position))
 
diff --git a/Tests/LocheckLogicTests/ValidateStringsTests.swift b/Tests/LocheckLogicTests/ValidateStringsTests.swift
index df4f9c7..2409226 100644
--- a/Tests/LocheckLogicTests/ValidateStringsTests.swift
+++ b/Tests/LocheckLogicTests/ValidateStringsTests.swift
@@ -181,4 +181,39 @@ class ValidateStringsTests: XCTestCase {
                 "abc:1: warning: 'missing' is missing from trnsltn (key_missing_from_translation)",
             ])
     }
+    
+    func testExtraKeyInTranslation() {
+        let problemReporter = ProblemReporter(log: false)
+        validateStrings(
+            baseStrings: [
+                LocalizedStringPair(
+                    string: "\"present\" = \"present\";",
+                    path: "abc",
+                    line: 0,
+                    basePath: "",
+                    baseLineFallback: 0)!,
+            ],
+            translationStrings: [
+                LocalizedStringPair(
+                    string: "\"present\" = \"tneserp\";",
+                    path: "def",
+                    line: 0,
+                    basePath: "",
+                    baseLineFallback: 0)!,
+                LocalizedStringPair(
+                    string: "\"extra\" = \"%s\";",
+                    path: "abc",
+                    line: 1,
+                    basePath: "",
+                    baseLineFallback: 0)!,
+            ],
+            baseLanguageName: "en",
+            translationLanguageName: "trnsltn",
+            problemReporter: problemReporter)
+
+        XCTAssertEqual(
+            problemReporter.problems.map(\.messageForXcode), [
+                "abc:1: warning: 'extra' is missing from the base translation (key_missing_from_base)",
+            ])
+    }
 }