diff --git a/src/runtime/javascript/ffi.ts b/src/runtime/javascript/ffi.ts
index 59ca2bcf2..14c19e4c3 100644
--- a/src/runtime/javascript/ffi.ts
+++ b/src/runtime/javascript/ffi.ts
@@ -106,8 +106,8 @@ const PgfMarshallerPtr = ref.refType(PgfMarshaller)
 // FFI
 
 export const runtime = ffi.Library('libpgf', {
-  pgf_read_pgf: [PgfDBPtr, [ref.types.CString, PgfRevisionPtr, PgfProbsCallbackPtr, PgfExnPtr]],
-  pgf_boot_ngf: [PgfDBPtr, [ref.types.CString, ref.types.CString, PgfRevisionPtr, PgfProbsCallbackPtr, PgfExnPtr]],
+  pgf_read_pgf: [PgfDBPtr, [ref.types.CString, PgfRevisionPtr, PgfProbsCallbackPtr | null, PgfExnPtr]],
+  pgf_boot_ngf: [PgfDBPtr, [ref.types.CString, ref.types.CString, PgfRevisionPtr, PgfProbsCallbackPtr | null, PgfExnPtr]],
   pgf_read_ngf: [PgfDBPtr, [ref.types.CString, PgfRevisionPtr, PgfExnPtr]],
   pgf_new_ngf: [PgfDBPtr, [PgfTextPtr, ref.types.CString, PgfRevisionPtr, PgfExnPtr]],
   pgf_write_pgf: [ref.types.void, [ref.types.CString, PgfDBPtr, PgfRevision, PgfExnPtr]],