refactor: add explicit rank check

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---

Author: kgryte

lib/node_modules/@stdlib/blas/ext/join/lib/assign.js

@@ -115,7 +115,7 @@ function assign( x, out, options ) {
 	}
 	// Broadcast the separator to match the shape of the non-reduced dimensions...
 	if ( isndarrayLike( s ) ) {
-		// When not provided `dims`, the operation is performed across all dimensions and `s` is assumed to be a zero-dimensional ndarray; when `dims` is provided, we need to broadcast `s` to match the shape of the non-core dimensions...
+		// When not provided `dims`, the operation is performed across all dimensions and `s` must be a zero-dimensional ndarray; when `dims` is provided, we need to broadcast `s` to match the shape of the non-core dimensions...
 		if ( hasOwnProp( opts, 'dims' ) ) {
 			s = maybeBroadcastArray( s, nonCoreShape( getShape( x ), opts.dims ) ); // eslint-disable-line max-len
 		}

lib/node_modules/@stdlib/blas/ext/join/lib/main.js

@@ -108,9 +108,11 @@ function join( x, options ) {
 	}
 	// Broadcast the separator to match the shape of the non-reduced dimensions...
 	if ( isndarrayLike( s ) ) {
-		// When not provided `dims`, the operation is performed across all dimensions and `s` is assumed to be a zero-dimensional ndarray; when `dims` is provided, we need to broadcast `s` to match the shape of the non-core dimensions...
+		// When not provided `dims`, the operation is performed across all dimensions and `s` must be a zero-dimensional ndarray; when `dims` is provided, we need to broadcast `s` to match the shape of the non-core dimensions...
 		if ( hasOwnProp( opts, 'dims' ) ) {
 			s = maybeBroadcastArray( s, nonCoreShape( getShape( x ), opts.dims ) ); // eslint-disable-line max-len
+		} else if ( getShape( s ).length !== 0 ) {
+			throw new TypeError( format( 'invalid option. `%s` option must be a zero-dimensional ndarray.', 'sep' ) );
 		}
 	} else {
 		if ( hasOwnProp( opts, 'dims' ) ) {