diff --git a/source/de/anomic/kelondro/blob/BLOBHeapModifier.java b/source/de/anomic/kelondro/blob/BLOBHeapModifier.java
index 4b8823f8d..df440512d 100644
--- a/source/de/anomic/kelondro/blob/BLOBHeapModifier.java
+++ b/source/de/anomic/kelondro/blob/BLOBHeapModifier.java
@@ -116,11 +116,17 @@ public class BLOBHeapModifier extends HeapReader implements BLOB {
             try {
                 long start = System.currentTimeMillis();
                 String fingerprint = HeapWriter.fingerprintFileHash(this.heapFile);
-                free.dump(HeapWriter.fingerprintGapFile(this.heapFile, fingerprint));
+                if (fingerprint == null) {
+                    Log.logSevere("kelondroBLOBHeap", "cannot write a dump for " + heapFile.getName()+ ": fingerprint is null");
+                } else {
+                    free.dump(HeapWriter.fingerprintGapFile(this.heapFile, fingerprint));
+                }
                 free.clear();
                 free = null;
-                index.dump(HeapWriter.fingerprintIndexFile(this.heapFile, fingerprint));
-                Log.logInfo("kelondroBLOBHeap", "wrote a dump for the " + this.index.size() +  " index entries of " + heapFile.getName()+ " in " + (System.currentTimeMillis() - start) + " milliseconds.");
+                if (fingerprint != null) {
+                    index.dump(HeapWriter.fingerprintIndexFile(this.heapFile, fingerprint));
+                    Log.logInfo("kelondroBLOBHeap", "wrote a dump for the " + this.index.size() +  " index entries of " + heapFile.getName()+ " in " + (System.currentTimeMillis() - start) + " milliseconds.");
+                }
                 index.close();
                 index = null;
             } catch (IOException e) {
diff --git a/source/de/anomic/kelondro/blob/HeapReader.java b/source/de/anomic/kelondro/blob/HeapReader.java
index dc973c0c6..0b7613261 100644
--- a/source/de/anomic/kelondro/blob/HeapReader.java
+++ b/source/de/anomic/kelondro/blob/HeapReader.java
@@ -101,6 +101,10 @@ public class HeapReader {
         // look for an index dump and read it if it exist
         // if this is successfull, return true; otherwise false
         String fingerprint = HeapWriter.fingerprintFileHash(this.heapFile);
+        if (fingerprint == null) {
+            Log.logSevere("HeapReader", "cannot generate a fingerprint for " + this.heapFile + ": null");
+            return false;
+        }
         File fif = HeapWriter.fingerprintIndexFile(this.heapFile, fingerprint);
         if (!fif.exists()) fif = new File(fif.getAbsolutePath() + ".gz");
         File fgf = HeapWriter.fingerprintGapFile(this.heapFile, fingerprint);
diff --git a/source/de/anomic/kelondro/blob/HeapWriter.java b/source/de/anomic/kelondro/blob/HeapWriter.java
index 55c476743..756b66b72 100644
--- a/source/de/anomic/kelondro/blob/HeapWriter.java
+++ b/source/de/anomic/kelondro/blob/HeapWriter.java
@@ -120,6 +120,7 @@ public final class HeapWriter  {
         assert f.exists() : "file = " + f.toString();
         String fp = Digest.fastFingerprintB64(f, false);
         assert fp != null : "file = " + f.toString();
+        if (fp == null) return null;
         return fp.substring(0, 12);
     }
     
@@ -156,9 +157,13 @@ public final class HeapWriter  {
             try {
                 long start = System.currentTimeMillis();
                 String fingerprint = HeapWriter.fingerprintFileHash(this.heapFileREADY);
-                new Gap().dump(fingerprintGapFile(this.heapFileREADY, fingerprint));
-                index.dump(fingerprintIndexFile(this.heapFileREADY, fingerprint));
-                Log.logInfo("kelondroBLOBHeapWriter", "wrote a dump for the " + this.index.size() +  " index entries of " + heapFileREADY.getName()+ " in " + (System.currentTimeMillis() - start) + " milliseconds.");
+                if (fingerprint == null) {
+                    Log.logSevere("kelondroBLOBHeapWriter", "cannot write a dump for " + heapFileREADY.getName()+ ": fingerprint is null");
+                } else {
+                    new Gap().dump(fingerprintGapFile(this.heapFileREADY, fingerprint));
+                    index.dump(fingerprintIndexFile(this.heapFileREADY, fingerprint));
+                    Log.logInfo("kelondroBLOBHeapWriter", "wrote a dump for the " + this.index.size() +  " index entries of " + heapFileREADY.getName()+ " in " + (System.currentTimeMillis() - start) + " milliseconds.");
+                }
                 index.close();
                 index = null;
             } catch (IOException e) {
diff --git a/source/de/anomic/kelondro/order/Digest.java b/source/de/anomic/kelondro/order/Digest.java
index 4f9407dc9..987b3bc10 100644
--- a/source/de/anomic/kelondro/order/Digest.java
+++ b/source/de/anomic/kelondro/order/Digest.java
@@ -249,6 +249,7 @@ public class Digest {
         try {
             byte[] b = fastFingerprintRaw(file, includeDate);
             assert b != null : "file = " + file.toString();
+            if (b == null || b.length == 0) return null;
             assert b.length != 0 : "file = " + file.toString();
             return Base64Order.enhancedCoder.encode(b);
         } catch (IOException e) {