diff --git a/build.ps1 b/build.ps1
index a963cf0..ae5de41 100644
--- a/build.ps1
+++ b/build.ps1
@@ -104,6 +104,34 @@ if ($Bootstrap) {
             throw 'Could not register the PSGallery repository; build dependencies cannot be installed.'
         }
 
+        # A repository named PSGallery is not necessarily the PowerShell Gallery. If one
+        # is already registered against a different SourceLocation, every dependency in
+        # build.depend.psd1 would be installed from it on the strength of the name alone.
+        #
+        # Compare the parsed URI's scheme, host and port rather than the string:
+        #
+        #   - String equality with -ne is case-insensitive, so a differently cased value
+        #     slipped through. Switching to -cne would be wrong in the other direction,
+        #     since host names are case-insensitive by definition and -cne would reject
+        #     https://WWW.PowerShellGallery.com/api/v2, which is the real gallery.
+        #   - Port matters: a non-default port on the same name reaches a different
+        #     listener. [Uri] supplies 443 for https when none is given, so the canonical
+        #     URL and an explicit :443 both match, while :444 does not.
+        #   - The path is deliberately not compared. The gallery serves both /api/v2 and
+        #     /api/v3, and any path on the genuine host is still the genuine host. Host
+        #     and port are the trust boundary; pinning the path would only add a false
+        #     rejection for a legitimate registration.
+        $expectedSource = [Uri]'https://www.powershellgallery.com/api/v2'
+        $actualSource = $psGallery.SourceLocation -as [Uri]
+        $sourceIsExpected = $null -ne $actualSource -and
+            $actualSource.Scheme -eq $expectedSource.Scheme -and
+            $actualSource.Host -eq $expectedSource.Host -and
+            $actualSource.Port -eq $expectedSource.Port
+        if (-not $sourceIsExpected) {
+            $expectedAuthority = '{0}://{1}:{2}' -f $expectedSource.Scheme, $expectedSource.Host, $expectedSource.Port
+            throw "The repository named 'PSGallery' points at [$($psGallery.SourceLocation)], which is not $expectedAuthority. Refusing to install build dependencies from an unexpected source."
+        }
+
         if ($psGallery.InstallationPolicy -ne 'Trusted') {
             Set-PSRepository -Name 'PSGallery' -InstallationPolicy 'Trusted'
         }