[imageMagick] Conversion works on command line but not through GM

[alexgurr]

We've installed imageMagick on the command line. We can manually convert between all required formats.

When we try to convert to/from PNGs through GM we get the following error

{ Error: Command failed: convert: NoDecodeDelegateForThisImageFormat `PNG' @ error/constitute.c/ReadImage/509.

18:05:37 0|app      | convert: NoImagesDefined `/opt/branding-portal/releases/branding-portal-48.0.4/assets-output/converted/logo.png' @ error/convert.c/ConvertImageCommand/3275.

18:05:37 0|app      |     at ChildProcess.onExit (/opt/branding-portal/releases/branding-portal-48.0.4/node_modules/gm/lib/command.js:301:17)

18:05:37 0|app      |     at emitTwo (events.js:126:13) 
18:05:37 0|app      |     at ChildProcess.emit (events.js:214:7) 
18:05:37 0|app      |     at maybeClose (internal/child_process.js:925:16) 
18:05:37 0|app      |     at Process.ChildProcess._handle.onexit (internal/child_process.js:209:5) code: 1, signal: null } [ '',

Listing the delegates on the command line correctly shows PNG. We've set the subClass in GM to imageMagic, so no graphicsMagick conflicts.

Any ideas?