diff --git a/src/__tests__/parser.test.ts b/src/__tests__/parser.test.ts
index e0378fb..e59d9dc 100644
--- a/src/__tests__/parser.test.ts
+++ b/src/__tests__/parser.test.ts
@@ -1,5 +1,6 @@
 import { describe, it, expect } from 'vitest';
 import { parse, detectFormat } from '../parser.js';
+import { diff } from '../diff.js';
 
 const cyclonedxFixture = {
   bomFormat: 'CycloneDX',
@@ -119,6 +120,34 @@ describe('parse (SPDX)', () => {
     expect(sbom.components[0].version).toBe('2.28.0');
     expect(sbom.components[0].license).toBe('Apache-2.0');
   });
+
+  it('treats NOASSERTION/NONE version and supplier sentinels as undefined', () => {
+    const sbom = parse({
+      spdxVersion: 'SPDX-2.3',
+      name: 'my-service',
+      packages: [
+        { name: 'internal-lib', versionInfo: 'NOASSERTION', supplier: 'NOASSERTION' },
+        { name: 'other-lib', versionInfo: 'NONE', supplier: 'Organization: Acme' },
+      ],
+    });
+    expect(sbom.components[0].version).toBeUndefined();
+    expect(sbom.components[0].supplier).toBeUndefined();
+    expect(sbom.components[1].version).toBeUndefined();
+    expect(sbom.components[1].supplier).toBe('Organization: Acme');
+  });
+
+  it('does not report a spurious upgrade when the old version is NOASSERTION', () => {
+    const old = parse({
+      spdxVersion: 'SPDX-2.3',
+      packages: [{ name: 'internal-lib', versionInfo: 'NOASSERTION' }],
+    });
+    const next = parse({
+      spdxVersion: 'SPDX-2.3',
+      packages: [{ name: 'internal-lib', versionInfo: '2.0.0' }],
+    });
+    const report = diff(old, next);
+    expect(report.upgraded).toHaveLength(0);
+  });
 });
 
 describe('parse (JSON string input)', () => {
diff --git a/src/parser.ts b/src/parser.ts
index 6232fc2..d5f0be4 100644
--- a/src/parser.ts
+++ b/src/parser.ts
@@ -62,10 +62,10 @@ export function parseSPDX(obj: Record<string, unknown>): SBOM {
   const components: Component[] = packages.map((pkg: Record<string, unknown>) => ({
     purl: extractSPDXPurl(pkg),
     name: typeof pkg.name === 'string' ? pkg.name : 'unknown',
-    version: typeof pkg.versionInfo === 'string' ? pkg.versionInfo : undefined,
+    version: normalizeSPDXValue(pkg.versionInfo),
     license: typeof pkg.licenseConcluded === 'string' ? pkg.licenseConcluded : undefined,
     ecosystem: extractEcosystemFromPurl(extractSPDXPurl(pkg) ?? ''),
-    supplier: typeof pkg.supplier === 'string' ? pkg.supplier : undefined,
+    supplier: normalizeSPDXValue(pkg.supplier),
   }));
 
   return {
@@ -161,6 +161,26 @@ function extractCycloneDXTimestamp(metadata: Record<string, unknown>): string |
   return typeof metadata.timestamp === 'string' ? metadata.timestamp : undefined;
 }
 
+/**
+ * Normalize an SPDX string field, treating the spec's `NOASSERTION` and `NONE`
+ * sentinels as "no value" (undefined) rather than real data.
+ *
+ * These sentinels are extremely common in generator output (e.g. `versionInfo:
+ * "NOASSERTION"` when a version can't be determined). Storing them verbatim
+ * corrupts the diff: a package whose version is "NOASSERTION" in the old SBOM
+ * and "2.0.0" in the new one is reported as an upgrade `NOASSERTION → 2.0.0`,
+ * which is meaningless. Collapsing the sentinel to undefined lets the diff's
+ * "both versions known" guard correctly skip it.
+ *
+ * (License normalization is handled separately — see PR #32.)
+ */
+function normalizeSPDXValue(value: unknown): string | undefined {
+  if (typeof value !== 'string') return undefined;
+  const trimmed = value.trim();
+  if (trimmed === '' || trimmed === 'NOASSERTION' || trimmed === 'NONE') return undefined;
+  return trimmed;
+}
+
 function extractSPDXPurl(pkg: Record<string, unknown>): string | undefined {
   const refs = pkg.externalRefs;
   if (!Array.isArray(refs)) return undefined;