diff --git a/src/lib.rs b/src/lib.rs
index c428c80..d89ff75 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -6,7 +6,7 @@ mod units;
 use conversions::convert;
 use format::format;
 use parse::{parse, ParseError};
-use units::{MetricQuantity, NonMetric};
+use units::{Metric, MetricQuantity, NonMetric};
 
 pub fn run(input: &str) -> Result<String, String> {
 	let non_metric = match parse(input) {
@@ -32,7 +32,8 @@ pub fn run(input: &str) -> Result<String, String> {
 	let metric: Vec<MetricQuantity> = non_metric.clone().into_iter().map(convert).collect();
 
 	// Make sure the results of the conversions can be summed together
-	// This is the case if the units after conversion are the same
+	// This is the case if the units after conversion are the same and
+	// they are not temperature units (i.e. degrees Celsius)
 	let mut metric_unit = None;
 	for index in 0..metric.len() {
 		match metric_unit {
@@ -48,6 +49,9 @@ pub fn run(input: &str) -> Result<String, String> {
 			}
 		}
 	}
+	if metric.len() > 1 && metric_unit == Some(Metric::Celsius) {
+		return Err("Cannot sum together temperatures".to_string());
+	}
 
 	let amount = metric.into_iter().map(|quantity| { quantity.amount }).sum();
 
@@ -92,6 +96,7 @@ mod test {
 		assert_eq!(run("1"), Err("Expected a unit".to_string()));
 		assert_eq!(run(""), Err("Expected quantity or quantities to convert".to_string()));
 		assert_eq!(run("6 ft 1 lbs"), Err("Incompatible units: feet, pounds".to_string()));
+		assert_eq!(run("0 °F 0 °F"), Err("Cannot sum together temperatures".to_string()));
 	}
 
 	#[test]