diff --git a/microbat/src/main/microbat/util/MicroBatUtil.java b/microbat/src/main/microbat/util/MicroBatUtil.java
index 29399e105..28cddf2b9 100644
--- a/microbat/src/main/microbat/util/MicroBatUtil.java
+++ b/microbat/src/main/microbat/util/MicroBatUtil.java
@@ -219,7 +219,10 @@ public static String getSourceFolder(String cName, String projectName) {
 		ICompilationUnit unit = JavaUtil.findICompilationUnitInProject(cName, projectName);
 		IPath uri = unit.getResource().getFullPath();
 		String sourceFolderPath = IResourceUtils.getAbsolutePathOsStr(uri);
-		cName = cName.substring(0, cName.lastIndexOf(".")).replace(".", File.separator);
+		int lastIndexOfDot = cName.lastIndexOf(".");
+		if (lastIndexOfDot != -1) {
+			cName = cName.substring(0, lastIndexOfDot).replace(".", File.separator);
+		}
 		sourceFolderPath = sourceFolderPath.substring(0, sourceFolderPath.indexOf(cName) - 1);
 		return sourceFolderPath;
 	}