diff --git a/src/runtime/javascript/ffi.ts b/src/runtime/javascript/ffi.ts
index b174b99df..59ca2bcf2 100644
--- a/src/runtime/javascript/ffi.ts
+++ b/src/runtime/javascript/ffi.ts
@@ -33,6 +33,9 @@ const PgfDBPtr = ref.refType(PgfDB)
 const PgfRevision = ref.refType(ref.types.void)
 export const PgfRevisionPtr = ref.refType(PgfRevision)
 
+const PgfProbsCallback = ref.refType(ref.types.void)
+export const PgfProbsCallbackPtr = ref.refType(PgfProbsCallback)
+
 export const PgfExn = Struct({
   type: ref.types.int,
   code: ref.types.int,
@@ -103,8 +106,8 @@ const PgfMarshallerPtr = ref.refType(PgfMarshaller)
 // FFI
 
 export const runtime = ffi.Library('libpgf', {
-  pgf_read_pgf: [PgfDBPtr, [ref.types.CString, PgfRevisionPtr, PgfExnPtr]],
-  pgf_boot_ngf: [PgfDBPtr, [ref.types.CString, ref.types.CString, PgfRevisionPtr, PgfExnPtr]],
+  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_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]],
diff --git a/src/runtime/javascript/index.ts b/src/runtime/javascript/index.ts
index 6c78db85d..8e317fd85 100644
--- a/src/runtime/javascript/index.ts
+++ b/src/runtime/javascript/index.ts
@@ -180,7 +180,7 @@ class PGFGrammar {
 function readPGF (path: string): PGFGrammar {
   const rev = ref.alloc(PgfRevisionPtr)
   const err = new PgfExn()
-  const db = runtime.pgf_read_pgf(path, rev, err.ref())
+  const db = runtime.pgf_read_pgf(path, rev, null, err.ref())
   handleError(err)
   return new PGFGrammar(db, rev)
 }
@@ -188,7 +188,7 @@ function readPGF (path: string): PGFGrammar {
 function bootNGF (pgf_path: string, ngf_path: string): PGFGrammar {
   const rev = ref.alloc(PgfRevisionPtr)
   const err = new PgfExn()
-  const db = runtime.pgf_boot_ngf(pgf_path, ngf_path, rev, err.ref())
+  const db = runtime.pgf_boot_ngf(pgf_path, ngf_path, rev, null, err.ref())
   handleError(err)
   return new PGFGrammar(db, rev)
 }