diff --git a/src/androidTest/java/com/nutomic/syncthingandroid/test/syncthing/RestApiTest.java b/src/androidTest/java/com/nutomic/syncthingandroid/test/syncthing/RestApiTest.java
index 927cfaa3..c59983ee 100644
--- a/src/androidTest/java/com/nutomic/syncthingandroid/test/syncthing/RestApiTest.java
+++ b/src/androidTest/java/com/nutomic/syncthingandroid/test/syncthing/RestApiTest.java
@@ -91,6 +91,15 @@ public class RestApiTest extends AndroidTestCase {
         assertEquals("1 GiB/s", RestApi.readableTransferRate(getContext(), 8589934592L));
     }
 
+    @SmallTest
+    public void testConvertNotCrashing() {
+        long[] values = new long[]{-1, 0, 1, 2, 4, 8, 16, 1024, 2^10, 2^15, 2^20, 2^25, 2^30};
+        for (long l : values) {
+            assertNotSame("", RestApi.readableFileSize(getContext(), l));
+            assertNotSame("", RestApi.readableTransferRate(getContext(), l));
+        }
+    }
+
     @MediumTest
     public void testGetConnections() throws InterruptedException {
         final CountDownLatch latch = new CountDownLatch(1);
diff --git a/src/main/java/com/nutomic/syncthingandroid/syncthing/RestApi.java b/src/main/java/com/nutomic/syncthingandroid/syncthing/RestApi.java
index d04fd1be..8c372b51 100644
--- a/src/main/java/com/nutomic/syncthingandroid/syncthing/RestApi.java
+++ b/src/main/java/com/nutomic/syncthingandroid/syncthing/RestApi.java
@@ -568,13 +568,19 @@ public class RestApi implements SyncthingService.OnWebGuiAvailableListener,
     }
 
     /**
-     * Converts a number of bytes to a human readable transfer rate in bits (eg 100 Kb/s).
+     * Converts a number of bytes to a human readable transfer rate in bytes per second
+     * (eg 100 KiB/s).
      */
     public static String readableTransferRate(Context context, long bits) {
         final String[] units = context.getResources().getStringArray(R.array.transfer_rate_units);
-        if (bits <= 0) return "0 " + units[0];
         long bytes = bits / 8;
+        if (bytes <= 0) return "0 " + units[0];
         int digitGroups = (int) (Math.log10(bytes) / Math.log10(1024));
+        if (digitGroups < 0 || digitGroups > 4)
+            throw new ArrayIndexOutOfBoundsException("bits=" + Long.toString(bits) +
+                    ", bytes=" + Long.toString(bytes) +
+                    " (int) digitGroups=" + Integer.toString(digitGroups) +
+                    " (double) digitGroups=" + Double.toString(Math.log10(bytes) / Math.log10(1024)));
         return new DecimalFormat("#,##0.#")
                 .format(bytes / Math.pow(1024, digitGroups)) + " " + units[digitGroups];
     }