NoClassDefFoundError in BlackBerry

I have been struggling a few days with a strange error in BlackBerry. My app works fine in the emulator, but when you start it on BlackBerry Bold (software version 4.5) you get a NoClassDefFoundError. On BlackBerry Storm (software version 4.6) it works fine, but on Bold2 (version 5.0) again you get NoClassDefFoundError.

The documentation says:

Thrown if the Java Virtual Machine tries to load in the definition of a class (as part of a normal method call or as part of creating a new instance using the new expression) and no definition of the class could be found.

The searched-for class definition existed when the currently executing class was compiled, but the definition can no longer be found.

I have gone through all the changes since the last working version and I finally found it. I had added some fields like this:

fldPortHttp = new TextField("Port:", "", 20, Field.EDITABLE | BasicEditField.FILTER_NUMERIC);

This worked well, with no compilation errors and in the emulator, but caused the NoClassDefFoundError at runtime. The solution was simply to change to:

fldPortHttp = new BasicEditField("Port:", "", 20, Field.EDITABLE | BasicEditField.FILTER_NUMERIC);