Strange exception while executing foreach statement

Hi. Today I've faced to exception while executing this fragment of code:

IDictionary<String, UsbDevice> deviceList = GetUsbManager().DeviceList;
bool found = false;
foreach (string key in deviceList.Keys)
{
UsbDevice foundDev = deviceList[key];
//do sth with foundDev
}

[mono-rt] Stacktrace:
[mono-rt] 
[mono-rt]   at <unknown> <0xffffffff>
[mono-rt]   at (wrapper managed-to-native) object.wrapper_native_0x417aaea1 (intptr,intptr,intptr) <IL 0x00027, 0xffffffff>
[mono-rt]   at Android.Runtime.JNIEnv.CallBooleanMethod (intptr,intptr) [0x00040] in /Users/builder/data/lanes/monodroid-mlion-monodroid-4.10.1-branch/62077ce0/source/monodroid/src/Mono.Android/src/Runtime/JNIEnv.g.cs:206
[mono-rt]   at Java.Util.IIteratorInvoker.get_HasNext () [0x00030] in /Users/builder/data/lanes/monodroid-mlion-monodroid-4.10.1-branch/62077ce0/source/monodroid/src/Mono.Android/platforms/android-18/src/generated/Java.Util.IIterator.cs:89
[mono-rt]   at System.Linq.Extensions/<ToEnumerator_Dispose>c__Iterator81.MoveNext () [0x0007a] in /Users/builder/data/lanes/monodroid-mlion-monodroid-4.10.1-branch/62077ce0/source/monodroid/src/Mono.Android/src/System.Linq/Extensions.cs:58
[mono-rt]   at RTAT.UsbProxy.SearchRudReader () [0x00071] in c:\Users\kszalach\Documents\Projects\RTAT\RTAT\UsbProxy.cs:25
[mono-rt]   at RTAT.AccessController.SearchForRudReader () [0x00037] in c:\Users\kszalach\Documents\Projects\RTAT\RTAT\AccessController.cs:81
[mono-rt]   at RTAT.AccessController.<HandleOnRudReaderNotFound>b__0 (object) [0x00000] in c:\Users\kszalach\Documents\Projects\RTAT\RTAT\AccessController.cs:97
[mono-rt]   at System.Threading.Timer/Scheduler.TimerCB (object) <IL 0x00013, 0x0015f>
[mono-rt]   at (wrapper runtime-invoke) <Module>.runtime_invoke_void__this___object (object,intptr,intptr,intptr) <IL 0x00052, 0xffffffff>
[mono-rt] 
[mono-rt] =================================================================
[mono-rt] Got a SIGABRT while executing native code. This usually indicates
[mono-rt] a fatal error in the mono runtime or one of the native libraries 
[mono-rt] used by your application.
[mono-rt] =================================================================
[mono-rt]

at RTAT.UsbProxy.SearchRudReader () [0x00071] in c:\Users\kszalach\Documents\Projects\RTAT\RTAT\UsbProxy.cs:25 means exactly foreach (string key in deviceList.Keys)

Any idea what and why it happened?