diff --git a/prism/src/prism/PrismFileLog.java b/prism/src/prism/PrismFileLog.java
index 88716299..3a7a8489 100644
--- a/prism/src/prism/PrismFileLog.java
+++ b/prism/src/prism/PrismFileLog.java
@@ -132,13 +132,23 @@ public class PrismFileLog extends PrismLog
 	@Override
 	public void flush()
 	{
+		if (fp == 0) {
+			throw new IllegalStateException("Trying to flush an invalid file handle (already closed?)");
+		}
 		PrismNative.PN_FlushFile(fp);
 	}
 
 	@Override
 	public void close()
 	{
+		if (fp == 0) {
+			// already closed, ignore (as specified by Closable contract)
+			return;
+		}
+
 		if (!stdout) PrismNative.PN_CloseFile(fp);
+		// set fp to zero to indicate that the file handle is not valid anymore
+		fp = 0;
 	}
 	
 	@Override
@@ -200,6 +210,10 @@ public class PrismFileLog extends PrismLog
 	 */
 	private void printToLog(String s)
 	{
+		if (fp == 0) {
+			throw new IllegalStateException("Trying to write to an invalid file handle (already closed?)");
+		}
+
 		PrismNative.PN_PrintToFile(fp, s);
 	}
 }
diff --git a/prism/src/prism/PrismLog.java b/prism/src/prism/PrismLog.java
index 57d81dac..979e9655 100644
--- a/prism/src/prism/PrismLog.java
+++ b/prism/src/prism/PrismLog.java
@@ -92,6 +92,7 @@ public abstract class PrismLog implements Closeable, AutoCloseable
 
 	public abstract void flush();
 
+	@Override
 	public abstract void close();
 
 	public abstract void print(boolean b);