diff --git a/dvc/command/diff.py b/dvc/command/diff.py
index 9e43709b4e..e68c269acf 100644
--- a/dvc/command/diff.py
+++ b/dvc/command/diff.py
@@ -69,7 +69,7 @@ def run(self):
             print(self._format(diff))
 
         except DvcException:
-            logger.exception("failed to get 'diff {}'")
+            logger.exception("failed to get diff")
             return 1
         return 0
 
diff --git a/dvc/repo/diff.py b/dvc/repo/diff.py
index a1ec334c03..89676f24b1 100644
--- a/dvc/repo/diff.py
+++ b/dvc/repo/diff.py
@@ -1,7 +1,9 @@
 import collections
 import os
 
+from dvc.exceptions import DvcException
 from dvc.repo import locked
+from dvc.scm.git import Git
 
 
 Diffable = collections.namedtuple("Diffable", "filename, checksum")
@@ -54,6 +56,9 @@ def diff(self, a_ref="HEAD", b_ref=None, *, target=None):
     the concept of `index`, but it keeps the same interface, thus,
     `dvc diff` would be the same as `dvc diff HEAD`.
     """
+    if type(self.scm) is not Git:
+        raise DvcException("only supported for Git repositories")
+
     outs = {}
 
     for branch in self.brancher(revs=[a_ref, b_ref]):
diff --git a/dvc/scm/base.py b/dvc/scm/base.py
index 412731ca89..e35c7b7d7d 100644
--- a/dvc/scm/base.py
+++ b/dvc/scm/base.py
@@ -29,7 +29,7 @@ def __init__(self, url, path):
 class RevError(SCMError):
     def __init__(self, url, rev):
         super().__init__(
-            "Failed to access revision '{}' for repo '{}'".format(rev, url)
+            "failed to access revision '{}' for repo '{}'".format(rev, url)
         )
 
 
diff --git a/dvc/scm/git/__init__.py b/dvc/scm/git/__init__.py
index 2d334a6493..8efb2dc5de 100644
--- a/dvc/scm/git/__init__.py
+++ b/dvc/scm/git/__init__.py
@@ -363,7 +363,12 @@ def get_rev(self):
         return self.repo.git.rev_parse("HEAD")
 
     def resolve_rev(self, rev):
-        return self.repo.git.rev_parse(rev)
+        from git.exc import GitCommandError
+
+        try:
+            return self.repo.git.rev_parse(rev)
+        except GitCommandError as exc:
+            raise RevError(url=self.root_dir, rev=rev) from exc
 
     def close(self):
         self.repo.close()
diff --git a/tests/func/test_diff.py b/tests/func/test_diff.py
index f7bb34d086..cd58870003 100644
--- a/tests/func/test_diff.py
+++ b/tests/func/test_diff.py
@@ -6,6 +6,7 @@
 import colorama
 
 from dvc.main import main
+from dvc.exceptions import DvcException
 
 
 def digest(text):
@@ -15,7 +16,8 @@ def digest(text):
 def test_no_scm(tmp_dir, dvc):
     tmp_dir.dvc_gen("file", "text")
 
-    pytest.skip("TODO: define behavior, should it fail?")
+    with pytest.raises(DvcException, match=r"only supported for Git repos"):
+        dvc.diff()
 
 
 def test_added(tmp_dir, scm, dvc):
@@ -86,7 +88,10 @@ def test_refs(tmp_dir, scm, dvc):
         ],
     }
 
-    pytest.skip('TODO: test dvc.diff("missing")')
+    with pytest.raises(
+        DvcException, match=r"failed to access revision 'missing'"
+    ):
+        dvc.diff("missing")
 
 
 def test_target(tmp_dir, scm, dvc):