diff --git a/lib/cli/options.js b/lib/cli/options.js
index b7779025b2..b25fe64db8 100644
--- a/lib/cli/options.js
+++ b/lib/cli/options.js
@@ -10,7 +10,12 @@
 const fs = require('fs');
 const ansi = require('ansi-colors');
 const yargsParser = require('yargs-parser');
-const {types, aliases} = require('./run-option-metadata');
+const {
+  types,
+  aliases,
+  isMochaFlag,
+  expectedTypeForFlag
+} = require('./run-option-metadata');
 const {ONE_AND_DONE_ARGS} = require('./one-and-dones');
 const mocharc = require('../mocharc.json');
 const {list} = require('./run-helpers');
@@ -18,7 +23,12 @@ const {loadConfig, findConfig} = require('./config');
 const findUp = require('find-up');
 const debug = require('debug')('mocha:cli:options');
 const {isNodeFlag} = require('./node-flags');
-const {createUnparsableFileError} = require('../errors');
+const {
+  createUnparsableFileError,
+  createInvalidArgumentTypeError,
+  createUnsupportedError
+} = require('../errors');
+const {isNumeric} = require('../utils');
 
 /**
  * The `yargs-parser` namespace
@@ -108,23 +118,21 @@ const parse = (args = [], defaultValues = {}, ...configObjects) => {
   // 3. to avoid explicitly defining the set of them, we tell yargs-parser they
   //    are ALL boolean flags.
   // 4. we can then reapply the values after yargs-parser is done.
-  const nodeArgs = (Array.isArray(args) ? args : args.split(' ')).reduce(
-    (acc, arg) => {
-      if (typeof arg !== 'string') {
-        throw new Error(`Invalid option ${arg} passed to mocha cli`);
-      }
-      const pair = arg.split('=');
-      let flag = pair[0];
-      if (isNodeFlag(flag, false)) {
-        flag = flag.replace(/^--?/, '');
-        return arg.includes('=')
-          ? acc.concat([[flag, pair[1]]])
-          : acc.concat([[flag, true]]);
-      }
-      return acc;
-    },
-    []
-  );
+  const allArgs = Array.isArray(args) ? args : args.split(' ');
+  const nodeArgs = allArgs.reduce((acc, arg) => {
+    if (typeof arg !== 'string') {
+      throw new Error(`Invalid option ${arg} passed to mocha cli`);
+    }
+    const pair = arg.split('=');
+    let flag = pair[0];
+    if (isNodeFlag(flag, false)) {
+      flag = flag.replace(/^--?/, '');
+      return arg.includes('=')
+        ? acc.concat([[flag, pair[1]]])
+        : acc.concat([[flag, true]]);
+    }
+    return acc;
+  }, []);
 
   const result = yargsParser.detailed(args, {
     configuration,
@@ -148,6 +156,30 @@ const parse = (args = [], defaultValues = {}, ...configObjects) => {
     result.argv[key] = value;
   });
 
+  const numericPositionalArgs = result.argv._.filter(arg => isNumeric(arg));
+  numericPositionalArgs.forEach(numericArg => {
+    const flag = allArgs
+      .map(arg => arg.replace(/^--?/, ''))
+      .find((arg, index) => {
+        return (
+          isMochaFlag(arg) &&
+          args[index + 1] === String(numericArg) &&
+          String(result.argv[arg]) !== String(numericArg)
+        );
+      });
+    if (flag) {
+      throw createInvalidArgumentTypeError(
+        `Flag ${flag} has invalid arg ${numericArg}`,
+        numericArg,
+        expectedTypeForFlag(flag)
+      );
+    } else {
+      throw createUnsupportedError(
+        `Invalid option ${numericArg} passed to mocha cli`
+      );
+    }
+  });
+
   return result.argv;
 };
 
@@ -195,8 +227,7 @@ const loadPkgRc = (args = {}) => {
           filepath
         );
       } else {
-        debug('failed to read default package.json at %s; ignoring',
-              filepath);
+        debug('failed to read default package.json at %s; ignoring', filepath);
         return result;
       }
     }
diff --git a/lib/cli/run-option-metadata.js b/lib/cli/run-option-metadata.js
index 83aa70dd7a..44ebf1e988 100644
--- a/lib/cli/run-option-metadata.js
+++ b/lib/cli/run-option-metadata.js
@@ -114,3 +114,23 @@ const ALL_MOCHA_FLAGS = Object.keys(TYPES).reduce((acc, key) => {
 exports.isMochaFlag = flag => {
   return ALL_MOCHA_FLAGS.has(flag.replace(/^--?/, ''));
 };
+
+/**
+ * Returns expected yarg option type for a given mocha flag.
+ * @param {string} flag - Flag to check (can be with out without leading "--"")
+ * @returns {string | undefined} - If flag is a valid mocha flag, the expected type of argument for this flag is returned, otherwise undefined is returned.
+ */
+exports.expectedTypeForFlag = flag => {
+  const normalizedName = flag?.replace(/^--?/, '');
+
+  // If flag is an alias, get it's full name.
+  const aliases = exports.aliases;
+  const fullFlagName =
+    Object.keys(aliases).find(flagName =>
+      aliases[flagName].includes(normalizedName)
+    ) || normalizedName;
+
+  return Object.keys(TYPES).find(flagType =>
+    TYPES[flagType].includes(fullFlagName)
+  );
+};
diff --git a/lib/utils.js b/lib/utils.js
index 31b313a6e0..89b21a32d6 100644
--- a/lib/utils.js
+++ b/lib/utils.js
@@ -689,3 +689,10 @@ exports.breakCircularDeps = inputObj => {
 
   return _breakCircularDeps(inputObj);
 };
+
+/**
+ * Checks if provided input can be parsed as a JavaScript Number.
+ */
+exports.isNumeric = input => {
+  return !isNaN(parseFloat(input));
+};