diff --git a/CHANGELOG.md b/CHANGELOG.md
index 51f1b2411..f78da1f2b 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -19,6 +19,7 @@ develop
 - Improve file encoding detection.
 - Update default priority order of encoding detection.
 - Improve encoding compatibility check.
+- Better error message on file opening.
 
 
 ### Fixes
diff --git a/CotEditor/Sources/NSString+CEEncoding.m b/CotEditor/Sources/NSString+CEEncoding.m
index 0a3559751..3d630fbe1 100644
--- a/CotEditor/Sources/NSString+CEEncoding.m
+++ b/CotEditor/Sources/NSString+CEEncoding.m
@@ -129,6 +129,10 @@ const char kUTF32LEBom[4] = {0xFF, 0xFE, 0x00, 0x00};
         }
     }
     
+    if (outError) {
+        *outError = [NSError errorWithDomain:NSCocoaErrorDomain code:NSFileReadUnknownStringEncodingError userInfo:nil];
+    }
+    
     *usedEncoding = NSNotFound;
     return nil;
 }
diff --git a/Tests/EncodingDetectionTests.swift b/Tests/EncodingDetectionTests.swift
index d394cdaf6..4c5416a8d 100644
--- a/Tests/EncodingDetectionTests.swift
+++ b/Tests/EncodingDetectionTests.swift
@@ -102,7 +102,14 @@ class EncodingDetectionTests: XCTestCase {
         let data = NSData()
         
         var encoding: NSStringEncoding = 0
-        let string = try? NSString(data: data, suggestedCFEncodings: [], usedEncoding: &encoding)
+        var string: NSString?
+        do {
+            string = try NSString(data: data, suggestedCFEncodings: [], usedEncoding: &encoding)
+        } catch let error as NSError {
+            XCTAssertEqual(error.domain, NSCocoaErrorDomain)
+            XCTAssertEqual(error.code, NSFileReadUnknownStringEncodingError)
+            XCTAssertNotNil(error.localizedDescription)
+        }
         
         XCTAssertNil(string)
         XCTAssertEqual(Int(encoding), NSNotFound)