"panic! (the 'impossible' happened)" attempting to print the contents of a mutable vector in GHCi

Summary

I got an panic when trying to print the contents of a mutable vector while debugging using "toList $ freeze t" in GHCi (t is my vector).

The full message is:

[/home/anthony/haskell/SubStringSearching/app/Main.hs:32:7-17] >toList $ freeze t

<interactive>:39:17: error:<interactive>: panic! (the 'impossible' happened)
  (GHC version 8.6.5 for x86_64-unknown-linux):
        No skolem info:
  [s_I8aZ[rt]]
  Call stack:
      CallStack (from HasCallStack):
        callStackDoc, called at compiler/utils/Outputable.hs:1160:37 in ghc:Outputable
        pprPanic, called at compiler/typecheck/TcErrors.hs:2891:5 in ghc:TcErrors

Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug

[/home/anthony/haskell/SubStringSearching/app/Main.hs:32:7-17] >

Steps to reproduce

In GHCi set a breakpoint on line 37 of the attached source code Main.hs
Step through the code until it reaches Main.hs:32:7-17
Run the command: toList $ freeze t

Expected behavior

Not a panic, probably print the contents of t.

Environment

GHC version used: 8.6.5 for x86_64-unknown-linux

Optional:

Operating System: Ubuntu 19.04
System Architecture: