diff --git a/src/commands.rs b/src/commands.rs
index 8e27d53ac..6f1345ec6 100644
--- a/src/commands.rs
+++ b/src/commands.rs
@@ -812,6 +812,12 @@ fn resolve_single_op_from_store(
     op_heads_store: &Arc<OpHeadsStore>,
     op_str: &str,
 ) -> Result<Operation, CommandError> {
+    if op_str.is_empty() || !op_str.as_bytes().iter().all(|b| b.is_ascii_hexdigit()) {
+        return Err(CommandError::UserError(format!(
+            "Operation ID \"{}\" is not a valid hexadecimal prefix",
+            op_str
+        )));
+    }
     if let Ok(binary_op_id) = hex::decode(op_str) {
         let op_id = OperationId::new(binary_op_id);
         match op_store.read_operation(&op_id) {
diff --git a/tests/test_operations.rs b/tests/test_operations.rs
index 08e4dbb75..c5195a792 100644
--- a/tests/test_operations.rs
+++ b/tests/test_operations.rs
@@ -48,7 +48,7 @@ fn test_op_log() {
     "###);
 
     // We get a reasonable message if an invalid operation ID is specified
-    insta::assert_snapshot!(test_env.jj_cmd_failure(&repo_path, &["log", "--at-op", "foo"]), @r###"Error: No operation ID matching "foo"
+    insta::assert_snapshot!(test_env.jj_cmd_failure(&repo_path, &["log", "--at-op", "foo"]), @r###"Error: Operation ID "foo" is not a valid hexadecimal prefix
 "###);
     // Odd length
     insta::assert_snapshot!(test_env.jj_cmd_failure(&repo_path, &["log", "--at-op", "123456789"]), @r###"Error: No operation ID matching "123456789"
@@ -57,8 +57,7 @@ fn test_op_log() {
     insta::assert_snapshot!(test_env.jj_cmd_failure(&repo_path, &["log", "--at-op", "0123456789"]), @r###"Error: No operation ID matching "0123456789"
 "###);
     // Empty ID
-    // TODO: This is not a reasonable message
-    insta::assert_snapshot!(test_env.jj_cmd_failure(&repo_path, &["log", "--at-op", ""]), @r###"Internal error: Failed to read operation: Other("IO error: Is a directory (os error 21)")
+    insta::assert_snapshot!(test_env.jj_cmd_failure(&repo_path, &["log", "--at-op", ""]), @r###"Error: Operation ID "" is not a valid hexadecimal prefix
 "###);
 }