Haskell stack fails with error message "unsatisfied constraints" - haskell

Question

When building my project, I'm receiving a cryptic error
C:\Users\T60\Dropbox\note-build>stack build
note-build-0.1.0.0: build
Preprocessing library note-build-0.1.0.0...
setup-Simple-Cabal-1.22.5.0-ghc-7.10.3.exe: unsatisfied constraints
-- While building package note-build-0.1.0.0 using:
C:\stack_root\setup-exe-cache\i386-windows\setup-Simple-Cabal-1.22.5.0-ghc-7.10.3.exe --builddir=.stack-work\dist\95439361 build lib:note-build exe:note-build --ghc-options " -ddump-hi -ddump-to-file"
Process exited with code: ExitFailure 1
It seems to be caused by adding filepath to the executable depends section of my cabal file. My Main.hs file uses System.FilePath, and without the line in my cabal file it fails, but adding it gives me this error. Prior to trying to use the filepath module, the project would build.
executable note-build
hs-source-dirs: app
main-is: Main.hs
ghc-options: -threaded -rtsopts -with-rtsopts=-N
build-depends: base
, filepath
, directory
, note-build
default-language: Haskell2010
What does this error mean, and am I putting the dependency in the wrong place in my cabal file?
Update: I ran the build using the -v flag, and it revealed that I'd absentmindedly substituted + for ++.
C:\Users\T60\Dropbox\note-build\app\Main.hs:20:31: #(stack_9kewhubnl5WIl89fhd1ea2:Stack.Build.Execute src/Stack\Build\Execute.hs:911:67)
2016-07-07 22:08:24.020516: [warn] No instance for (Num FilePath) arising from a use of `+' #(stack_9kewhubnl5WIl89fhd1ea2:Stack.Build.Execute src/Stack\Build\Execute.hs:911:67)
2016-07-07 22:08:24.051760: [warn] In the first argument of `normalise', namely `(y + "/")' #(stack_9kewhubnl5WIl89fhd1ea2:Stack.Build.Execute src/Stack\Build\Execute.hs:911:67)
2016-07-07 22:08:24.067386: [warn] In the expression: normalise (y + "/") #(stack_9kewhubnl5WIl89fhd1ea2:Stack.Build.Execute src/Stack\Build\Execute.hs:911:67)
2016-07-07 22:08:24.129886: [warn] In an equation for `y_norm': y_norm = normalise (y + "/").
So the fix was easy enough, but should I consider this a potential issue with stack that I had to use verbose output to see what seems like a simple type error?