diff --git a/src/validators/SourceMapFormatValidator.ts b/src/validators/SourceMapFormatValidator.ts
index 75caca3..d245798 100644
--- a/src/validators/SourceMapFormatValidator.ts
+++ b/src/validators/SourceMapFormatValidator.ts
@@ -2,54 +2,71 @@ import { Validator } from "../util/Validator.js";
 import { ValidationResult } from "../util/ValidationResult.js";
 import type { ValidationContext } from "../util/ValidationContext.js";
 
-export class SourceMapFormatValidator extends Validator {
-  validate({ sourceMap }: ValidationContext): ValidationResult {
-    const errors: Error[] = [];
+// Defined separately from the validator class below in order to enable easier
+// recursive calls.
+function validateSourceMap(sourceMap : any) : Error[] {
+  const errors: Error[] = [];
 
-    if (sourceMap.version !== 3) {
-      errors.push(new Error("Source map version is not 3."));
+  if (sourceMap.version !== 3) {
+    errors.push(new Error(`Source map version is not 3, got ${sourceMap.version}.`));
+  }
+
+  if ("sections" in sourceMap) {
+    if ("mappings" in sourceMap) {
+      errors.push(new Error('Source map cannot have both "mappings" and "sections" fields.'));
     }
 
-    if ("sections" in sourceMap) {
-      if ("mappings" in sourceMap) {
-        errors.push(new Error('Source map cannot have both "mappings" and "sections" fields.'));
-      }
+    if (typeof sourceMap.sections !== "object") {
+      errors.push(new Error('Source map "sections" field is not an object.'));
+    } else {
+      sourceMap.sections.forEach((section: any, i: number) => {
+        if (!section.map || !(typeof section.map === "object" && section.map !== null)) {
+          errors.push(new Error(`Index map section ${i} is missing a "map" field or it is invalid.`));
+        } else {
+          // NB: this allows arbitrarily nested index maps, but is that
+          // intended by the spec?
+          const subErrors = validateSourceMap(section.map);
+          errors.push(...subErrors);
+        }
+      });
+    }
+  } else {
+    if (!sourceMap.sources || !Array.isArray(sourceMap.sources)) {
+      errors.push(new Error('Source map "sources" field is invalid or missing.'));
+    } else {
+      sourceMap.sources.forEach((x: unknown, i: number) => {
+        if (typeof x !== "string" && x !== null) errors.push(new Error(`There is a source with an invalid format on the index ${i}. Each source should be defined as a string or null`))
+      })
+    }
 
-      if (typeof sourceMap.sections !== "object") {
-        errors.push(new Error('Source map "sections" field is not an object.'));
-      }
+    if (!sourceMap.names || !Array.isArray(sourceMap.names)) {
+      errors.push(new Error('Source map "names" field is missing.'));
     } else {
-      if (!sourceMap.sources || !Array.isArray(sourceMap.sources)) {
-        errors.push(new Error('Source map "sources" field is invalid or missing.'));
-      } else {
-        sourceMap.sources.forEach((x: unknown, i: number) => {
-          if (typeof x !== "string" && x !== null) errors.push(new Error(`There is a source with an invalid format on the index ${i}. Each source should be defined as a string or null`))
-        })
-      }
-
-      if (!sourceMap.names || !Array.isArray(sourceMap.names)) {
-        errors.push(new Error('Source map "names" field is missing.'));
-      } else {
-        sourceMap.names.forEach((x: unknown, i: number) => {
-          if (typeof x !== "string") errors.push(new Error(`There is a name with an invalid format on the index ${i}. Each name should be defined as a string`))
-        })
-      }
-
-      if (!("mappings" in sourceMap)) {
-        errors.push(new Error('Source map "mappings" field is missing.'));
-      } else if (typeof sourceMap.mappings !== "string") {
-        errors.push(new Error('Source map "mappings" field is not a string.'));
-      }
-
-      if ('sourcesContent' in sourceMap) {
-        if (!Array.isArray(sourceMap.sourcesContent))
-          errors.push(new Error('Source map "sources" field is invalid.'));
-        sourceMap.sourcesContent.forEach((x: unknown, i: number) => {
-          if (x !== null && typeof x !== "string") errors.push(new Error(`There is a source content with an invalid format on the index ${i}. Each content should be defined as a strings or null`))
-        })
-      }
+      sourceMap.names.forEach((x: unknown, i: number) => {
+        if (typeof x !== "string") errors.push(new Error(`There is a name with an invalid format on the index ${i}. Each name should be defined as a string`))
+      })
     }
 
-    return ValidationResult.from(errors);
+    if (!("mappings" in sourceMap)) {
+      errors.push(new Error('Source map "mappings" field is missing.'));
+    } else if (typeof sourceMap.mappings !== "string") {
+      errors.push(new Error('Source map "mappings" field is not a string.'));
+    }
+
+    if ('sourcesContent' in sourceMap) {
+      if (!Array.isArray(sourceMap.sourcesContent))
+        errors.push(new Error('Source map "sources" field is invalid.'));
+      sourceMap.sourcesContent.forEach((x: unknown, i: number) => {
+        if (x !== null && typeof x !== "string") errors.push(new Error(`There is a source content with an invalid format on the index ${i}. Each content should be defined as a strings or null`))
+      })
+    }
+  }
+
+  return errors;
+}
+
+export class SourceMapFormatValidator extends Validator {
+  validate({ sourceMap }: ValidationContext): ValidationResult {
+    return ValidationResult.from(validateSourceMap(sourceMap));
   }
 }