What does it mean that no cert given is a "domain name certificate"?

Question

I'm trying to add an SSL certificate to Heroku but I'm getting an error:

heroku certs:add domain.pem private.key
Resolving trust chain... failed
 !    No certificate given is a domain name certificate.

What does it mean that no certificate given is a domain name certificate?

Edit: here's the output of openssl x509 -text -in domain.pem (with the certificate itself omitted).

If it helps, I do currently have SSL set up for my domain, snipsalonsoftware.com. I want to move my application to app.snipsalonsoftware.com and it's the SSL certificate for the app subdomain that I'm trying to add now.

Certificate:
    Data:
        Version: 3 (0x2)
        Serial Number:
            6c:e8:89:ec:34:02:23:f6:0a:d7:e1:e3:e6:0a:a2:b7
        Signature Algorithm: sha1WithRSAEncryption
        Issuer: C=GB, ST=Greater Manchester, L=Salford, O=COMODO CA Limited, CN=PositiveSSL CA 2
        Validity
            Not Before: Mar 12 00:00:00 2014 GMT
            Not After : Mar 12 23:59:59 2015 GMT
        Subject: OU=Domain Control Validated, OU=Provided by New Dream Network, LLC, OU=DreamHost Basic SSL, CN=app.snipsalonsoftware.com
        Subject Public Key Info:
            Public Key Algorithm: rsaEncryption
            RSA Public Key: (2048 bit)
                Modulus (2048 bit):
                    00:f3:e3:db:d3:85:b0:67:57:9c:2e:3e:20:db:70:
                    77:9f:dd:7f:23:a9:f4:3f:8d:77:25:f0:5b:b7:4f:
                    a8:df:13:34:15:1a:82:5e:78:6a:1e:67:cd:71:f9:
                    15:9a:f1:a1:b3:e5:24:43:38:3c:be:76:d3:db:67:
                    fc:80:0d:8e:c0:d3:0b:30:12:08:62:81:be:87:6f:
                    a9:03:ce:24:59:cd:a3:8d:bf:8f:dc:80:6b:1b:cf:
                    05:ca:19:e9:28:26:70:ed:f0:67:6b:11:85:7d:bb:
                    64:16:3f:86:50:76:65:22:d0:b5:de:dd:66:cc:7d:
                    bc:89:d4:5b:d2:ee:1e:74:10:74:70:6e:c4:3d:b0:
                    51:96:63:9f:05:6e:d2:21:a0:74:b6:2c:40:82:35:
                    2e:5c:3f:12:1b:f5:5b:6e:40:f5:c5:02:a8:ef:a9:
                    db:df:7c:d4:88:2a:f7:ec:25:f6:b6:87:30:89:7b:
                    ba:5f:dd:66:34:ae:f8:0d:f4:20:19:85:26:91:83:
                    15:e4:9e:53:3c:ff:f5:10:37:6a:42:e7:d7:dc:29:
                    cd:5d:d4:07:a8:34:34:cf:9d:e5:bc:63:5b:d5:a1:
                    8d:ea:d4:3e:a9:ea:2f:25:49:e7:fb:1d:7a:e9:1d:
                    83:55:12:d7:a8:71:80:b5:b3:21:3c:41:54:cd:c5:
                    0e:f9
                Exponent: 65537 (0x10001)
        X509v3 extensions:
            X509v3 Authority Key Identifier: 
                keyid:99:E4:40:5F:6B:14:5E:3E:05:D9:DD:D3:63:54:FC:62:B8:F7:00:AC

            X509v3 Subject Key Identifier: 
                C6:BF:8B:52:90:D6:16:0B:54:54:27:AE:6B:8D:27:F1:03:AF:7E:96
            X509v3 Key Usage: critical
                Digital Signature, Key Encipherment
            X509v3 Basic Constraints: critical
                CA:FALSE
            X509v3 Extended Key Usage: 
                TLS Web Server Authentication, TLS Web Client Authentication
            X509v3 Certificate Policies: 
                Policy: 1.3.6.1.4.1.6449.1.2.2.7
                  CPS: http://www.positivessl.com/CPS
                Policy: 2.23.140.1.2.1

            X509v3 CRL Distribution Points: 
                URI:http://crl.comodoca.com/PositiveSSLCA2.crl

            Authority Information Access: 
                CA Issuers - URI:http://crt.comodoca.com/PositiveSSLCA2.crt
                OCSP - URI:http://ocsp.comodoca.com

            X509v3 Subject Alternative Name: 
                DNS:app.snipsalonsoftware.com, DNS:www.app.snipsalonsoftware.com
    Signature Algorithm: sha1WithRSAEncryption
        6c:4f:5c:16:a1:e5:e2:83:a2:84:d5:50:52:75:b8:3f:d2:b5:
        b4:3f:5b:01:2e:d9:a5:f9:cf:04:94:44:df:2d:57:72:75:00:
        4b:1c:0b:9d:ad:19:88:cf:c7:ab:37:50:df:4c:fa:c5:d0:db:
        3f:79:ae:ea:c8:84:6b:d3:10:a1:65:68:b4:ec:4e:f1:2a:f8:
        b2:87:db:7b:19:d8:e1:49:07:c7:eb:68:ae:58:84:d2:8d:67:
        b4:a8:11:cf:a2:c7:4f:5d:cc:4b:64:0f:7c:3b:9f:07:a3:3d:
        66:81:eb:08:61:c4:15:35:93:56:ea:f8:fe:87:58:12:a7:c8:
        25:d0:8e:eb:e2:fe:e3:8b:5b:f4:8a:55:09:0a:c1:3e:16:47:
        00:35:4b:7b:78:8d:2b:7e:4e:37:b9:c9:fd:4c:21:f4:09:50:
        66:2f:06:3b:3d:73:91:70:3e:16:89:01:22:6e:71:52:06:99:
        d5:9f:c2:f9:fc:97:9e:c6:73:cd:40:31:b6:34:1f:2d:aa:0f:
        6b:ba:9e:79:b9:90:79:d8:15:f4:17:bc:25:8e:23:06:13:6a:
        72:db:e5:e4:29:9a:7a:25:ba:38:ef:3e:a8:88:56:23:86:32:
        07:59:8b:02:37:20:65:65:34:c9:ae:c3:2f:c5:41:6f:6e:cc:
        8b:ac:01:c5