diff --git a/libse/SubtitleFormats/Dost.cs b/libse/SubtitleFormats/Dost.cs
index 1ba852afb..ac2a0005d 100644
--- a/libse/SubtitleFormats/Dost.cs
+++ b/libse/SubtitleFormats/Dost.cs
@@ -36,7 +36,9 @@ namespace Nikse.SubtitleEdit.Core.SubtitleFormats
             if (!sb.ToString().Contains("$FORMAT"))
                 return false;
 
+            var oldFrameRate = Configuration.Settings.General.CurrentFrameRate;
             LoadSubtitle(subtitle, lines, fileName);
+            Configuration.Settings.General.CurrentFrameRate = oldFrameRate;
             return subtitle.Paragraphs.Count > _errorCount;
         }
 
diff --git a/libse/SubtitleFormats/FinalCutProTest2Xml.cs b/libse/SubtitleFormats/FinalCutProTest2Xml.cs
index 58dc86565..4a481f8fd 100644
--- a/libse/SubtitleFormats/FinalCutProTest2Xml.cs
+++ b/libse/SubtitleFormats/FinalCutProTest2Xml.cs
@@ -27,7 +27,9 @@ namespace Nikse.SubtitleEdit.Core.SubtitleFormats
         public override bool IsMine(List<string> lines, string fileName)
         {
             var subtitle = new Subtitle();
+            var oldFrameRate = Configuration.Settings.General.CurrentFrameRate;
             LoadSubtitle(subtitle, lines, fileName);
+            Configuration.Settings.General.CurrentFrameRate = oldFrameRate;
             return subtitle.Paragraphs.Count > 0;
         }
 
diff --git a/libse/SubtitleFormats/FinalCutProTextXml.cs b/libse/SubtitleFormats/FinalCutProTextXml.cs
index 38d36dc0d..87be59432 100644
--- a/libse/SubtitleFormats/FinalCutProTextXml.cs
+++ b/libse/SubtitleFormats/FinalCutProTextXml.cs
@@ -26,7 +26,9 @@ namespace Nikse.SubtitleEdit.Core.SubtitleFormats
         public override bool IsMine(List<string> lines, string fileName)
         {
             var subtitle = new Subtitle();
+            var oldFrameRate = Configuration.Settings.General.CurrentFrameRate;
             LoadSubtitle(subtitle, lines, fileName);
+            Configuration.Settings.General.CurrentFrameRate = oldFrameRate;
             return subtitle.Paragraphs.Count > 0;
         }
 
diff --git a/libse/SubtitleFormats/FinalCutProXml.cs b/libse/SubtitleFormats/FinalCutProXml.cs
index 09c826dac..bed74361e 100644
--- a/libse/SubtitleFormats/FinalCutProXml.cs
+++ b/libse/SubtitleFormats/FinalCutProXml.cs
@@ -27,7 +27,9 @@ namespace Nikse.SubtitleEdit.Core.SubtitleFormats
         public override bool IsMine(List<string> lines, string fileName)
         {
             var subtitle = new Subtitle();
+            var oldFrameRate = Configuration.Settings.General.CurrentFrameRate;
             LoadSubtitle(subtitle, lines, fileName);
+            Configuration.Settings.General.CurrentFrameRate = oldFrameRate;
             return subtitle.Paragraphs.Count > 0;
         }
 
diff --git a/libse/SubtitleFormats/IssXml.cs b/libse/SubtitleFormats/IssXml.cs
index ce0fcd4af..b8f0894a4 100644
--- a/libse/SubtitleFormats/IssXml.cs
+++ b/libse/SubtitleFormats/IssXml.cs
@@ -28,7 +28,9 @@ namespace Nikse.SubtitleEdit.Core.SubtitleFormats
         public override bool IsMine(List<string> lines, string fileName)
         {
             var subtitle = new Subtitle();
+            var oldFrameRate = Configuration.Settings.General.CurrentFrameRate;
             LoadSubtitle(subtitle, lines, fileName);
+            Configuration.Settings.General.CurrentFrameRate = oldFrameRate;
             return subtitle.Paragraphs.Count > 0;
         }
 
diff --git a/libse/SubtitleFormats/UnknownSubtitle41.cs b/libse/SubtitleFormats/UnknownSubtitle41.cs
index 1bd8e130c..01ee88e0c 100644
--- a/libse/SubtitleFormats/UnknownSubtitle41.cs
+++ b/libse/SubtitleFormats/UnknownSubtitle41.cs
@@ -26,7 +26,9 @@ namespace Nikse.SubtitleEdit.Core.SubtitleFormats
         public override bool IsMine(List<string> lines, string fileName)
         {
             Subtitle subtitle = new Subtitle();
+            var oldFrameRate = Configuration.Settings.General.CurrentFrameRate;
             LoadSubtitle(subtitle, lines, fileName);
+            Configuration.Settings.General.CurrentFrameRate = oldFrameRate;
             return subtitle.Paragraphs.Count > _errorCount;
         }
 
diff --git a/src/Test/Logic/SubtitleFormats/SubtitleFormatsTest.cs b/src/Test/Logic/SubtitleFormats/SubtitleFormatsTest.cs
index f62887b50..c13f49a2d 100644
--- a/src/Test/Logic/SubtitleFormats/SubtitleFormatsTest.cs
+++ b/src/Test/Logic/SubtitleFormats/SubtitleFormatsTest.cs
@@ -5,6 +5,7 @@ using System;
 using System.Collections.Generic;
 using System.Globalization;
 using System.Linq;
+using System.Text;
 using System.Xml;
 
 namespace Test.Logic.SubtitleFormats
@@ -828,5 +829,35 @@ Dialogue: Marked=0,0:00:01.00,0:00:03.00,Default,NTP,0000,0000,0000,!Effect," +
             Assert.AreEqual("#Every satellite...#", actual);
         }
         #endregion
+
+        #region Unknown format
+        [TestMethod]
+        public void TestUnknownFormatAndFrameRate()
+        {
+            var sb = new StringBuilder();
+            for (int i = 0; i < 500; i++)
+            {
+                sb.AppendLine(i.ToString(CultureInfo.InvariantCulture));
+                sb.AppendLine(string.Empty.PadLeft(50, '0'));
+                sb.AppendLine();
+            }
+            var lines = sb.ToString().SplitToLines().ToList();
+            Configuration.Settings.General.CurrentFrameRate = 27;
+            foreach (var format in SubtitleFormat.AllSubtitleFormats)
+            {
+                //if (format.IsTextBased)
+                {
+                    if (format.IsMine(lines, null))
+                    {
+                        Assert.Fail("This format should not be recognized");
+                    }
+                }
+            }
+            if (Math.Abs(Configuration.Settings.General.CurrentFrameRate - 27) > 0.01)
+            {
+                Assert.Fail("Frame rate changed in 'IsMine'!");
+            }
+        }
+        #endregion  
     }
 }