gulp can't find module source-map

Question

I'm having an issue with sourcemaps in gulp: when I try to implement gulp-sourcemaps directly through a pipe, but also when I try to feed it in via webpack, which if I've understood correctly, has sourcemaps by default, I keep getting the same confusing error:

Error: Cannot find module 'source-map'

I can see the @gulp-sourcemaps folder in my node_modules, which contains within it an identity-map folder containing a bunch more node_modules including the source-map module in question, so I think everything is hooked up as it should be. That being said, I'm new to gulp, so I might be missing something really obvious.

Can anyone offer me some guidance on how to help gulp find the module?

// package vars
const pkg = require("./package.json");

// gulp
const gulp = require("gulp");
const gulpIf = require("gulp-if");

// webpack
const webpack_config = require("./webpack.config");

// load all plugins in "devDependencies" into the variable $
const $ = require("gulp-load-plugins")({
  pattern: ["*"],
  scope: ["devDependencies"]
});

// ...

gulp.task("sass", function() {
  return gulp
    .src(pkg.paths.app.scss + "**/*.scss")
    .pipe($.sourcemaps.init())
    .pipe($.sass()) 
    .pipe($.autoprefixer())
    .pipe($.sourcemaps.write("./maps"))
    .pipe(gulp.dest(pkg.paths.app.css))
    .pipe(
      $.browserSync.reload({
        stream: true
      })
    );
});

// ... 

gulp.task("webpack", function() {
  return gulp
    .src(pkg.paths.app.js + "**/*.js")
    .pipe($.webpack(webpack_config))
    .pipe(gulp.dest(pkg.paths.public.js));
});

// ...